Sorry, I wasn't clear; I kind of glossed over that because I didn't want to get into temp and it's not really important ... it's basically to address a peculiarity of the way that the machine works ...
There are four main phases in Rao's blooming espresso profile. The phases and the instructions to the machine are basically:
Preinfusion - "Give me water at 4ml/s until you detect the pressure spike that indicates the puck is saturated."
Pause (what I called "soak") - "Just turn the pump off and relax for a set time"
Ramp - "Give me water; ramp up over a set time"
Flow - "Maintain 2.2ml/s flow rate until time is up"
(So maybe the real answer to your question is that I my reference to "soak" was vague and was not meant to be a reference to "preinfusion" ... but let's keep going because it's kind of interesting for people interested in the machine ...)
For each stage, you have to set a temperature. When you set the temperature, the machine mixes together water to hit that temperature. To stop all that effort from going to waste and losing the temperature to the group head, there is a cartridge heater in the group head that targets your target temperature.
So Rao sets the preinfusion phase to a fair bit higher than the flowing brew temperature, since the first lot of water that goes into the puck experiences a temperature drop due to the heat absorption by the dry ground coffee. Then the balance of the shot, the ramp and flow stages, have the water come through at a lower brew temp, since that water doesn't need to do as much work to compensate for the initial temp drop (so goes the theory ... somewhat borne out by the reading from the temp probe immediately behind the shower screen, but probably needs some Scace device verification ... no doubt someone has already done it; probably Scott). In other words, if you set a flat brew temp, the water in temp in the puck will asymptote towards the target, but for much of the beginning of the brew, you won't hit the target. If you preheat the preinfusion water more, you can get the brew to the target faster across the shot.
So far, so good. But what about the soak phase? There's no water flowing, so no need to worry about temperature, right? Wrong! Because the machine has that cartridge heater to keep the group toasty, and that thing has to be set to some value. Now if you leave it set to the preinfusion temp, then the water temp that you record will always overshoot the target. Because the cartridge heater seems to have a fairly big effect on it. If you set the temperature at the soak phase to the target temp for the ramp phase, you will still overshoot. Because the group is full of hot water that came in at the preinfusion temperature, so the system's capacity to lose heat is limited. So in the soak phase, I program the target temperature to undershoot what I want in the ramp and flow phases so that what I actually get during those phases is pretty close to the target.
Here's a graph to illustrate it. What you need to know is the solid line is the current shot, the dotted line is the target temp, the vertical black bars indicate moving on to the next stage and the pale solid red line is the reference shot I had displayed, which is irrelevant to this discussion:
... so basically you can see that I preinfused at 91.5C, set the heater target to 88C during the soak phase and set 89.5C as the target for the remainder of the brew. (For completeness, I should note that there's actually a final phase to the shot that resets the group heater to the preinfusion target value so that that's where it sits between shots ... well, at least until the machine hits your designated cooldown time ... I have it cool down really quickly to save energy, since it ramps up so quickly ... anyway, I massively digress ...) Looks like maybe if I bumped up the soak temp by 0.5C I might be able to hit the target even closer. I think it's kind of nice that the Decent tries its hardest to hit the target and is honest with you about how close it gets.
So when you say that people say that they use low brew temperatures for a blooming espresso shot, there are kind of two things at work here. First, the decent reads the temperature from a sensor immediately behind the shower screen, which basically sits within the brass dispersion block. This *seems* to result in lower readings for a similar in-cup result to most other machines that tell you the temperature at the point at which the probe measures the boiler temperature, and then subtract a programmed fixed offset. Second, in the case of Rao's blooming espresso shot, if they are talking about a single temperature, they are probably oversimplifying and it might be that the extra heat in the preheat makes it equivalent to a hotter temperature on other machines.
I think that means that I've kind of addressed the question about my approach in a roundabout way. This shot happened to be sort of a light espresso roast; I guess I would probably have set my Vesuvius to maybe 92 or 93C for this coffee, so, yes, I am going a little lower on the Decent. I'm not quite sure what temperature I'd use for the simpler "best overall" pressure profile that is much more like a regular machine; I'd probably start at maybe 90.5C for this coffee, I guess, as a first try. So, yes, I suspect that the ramp and brew stage blooming espresso temperature that I would use probably is a bit lower than the single brew temperature on single stage profiles. But I've only had the machine for about a month, so I don't really feel that I have enough experience with it to comment. What's nice is that I'm already very happy with the shots that I'm pulling.
Now, for the lightning round! Why should we have an undershoot in the soak phase? Does that get the coffee to taste better? My answer: no idea, yet. I'm pragmatically putting the undershoot in in the hope that the targets that I set will be easier for me to correlate with the results in the cup, which I assume will be much harder to do if it's not WYSIWYG by virtue of the temperature distortion of the group heater during the soak phase. We've had many discussions on this site about whether the flatline brew water profile of a LM saturated group is better than the HX hump and I agree that the fact that a temp profile is flat is not necessarily better. At some stage, I'll give some temperature profiling a shot. But for the moment with me getting used to a new machine and hitherto non-existent functionality, I wanted to sort of stick brew temperature profiling in a box for the moment to focus on all of the other moving parts. I think it's also worthwhile noting all of the discussions we have had about the difference in thermocouple readings in roasters and the effect of the thermocouple shield size and their placement, and the idea that readings from one roaster aren't transferrable to others. We may well be experiencing temperature artifacts from the probe details in espresso machines, too. For example, if I'm right that the DE is graphing from the temp sensor that sits immediately behind the shower screen, it may well be that because it sits in a hole in the brass dispersion block, the dispersion block has some sort of tempering effect on the reading that isn't really reflective of what a bead probe would read if it was the massive distance of a cm away in the puck. As always, the temperature reading is only valid at one very specific point in any machine and the important thing is not that the absolute value is correct, but that it is repeatable and that we can correlate it with results.
PS. Tom, you might well have been across a lot of that stuff, but I thought it would be good to answer at length for the benefit of of others who might not have seen how the advanced programming mode works.