I stumbled upon the staccato shot and I started experimenting with it. I found an explanation on youtube, but I think it's lacking. They were saying that water contact time is the main factor. Assume that water flows at constant rate, yes, the top of the puck will get more contact time, but that water at the end will not end in your cup, so it doesn't matter. I do think there's a water contact time effect, but it's mostly if you use preinfusion.

To me, the effect of the staccato is for the following reasons:

1. main effect I think it's pressure differences at the top of the puck and the bottom of the puck. The top will be hit with say 9 bars of pressure, but because of puck resistance, the bottom will have much less pressure, otherwise the water will come out at the flow of the machine
2. temperature differences. This difference will be at the beginning of the shot. Temperature differences can only explain a small part of the effect
3. water contact time. This is mostly during the pre-infusion.
4. less chance of channeling. Machine gets clogged due to too much puck resistance, which is mostly the fines, but they are at the bottom where there's less pressure, hence less channeling.

Did anybody experiment with different pressures for the staccato shot? Can you even go beyond 9 bars?

Another thing, I noticed somebody doing staccato with the coarse in the middle. This doesn't make sense to me at all. Maybe there's a fines migration effect going on, but still...

The staccato shot clearly extracts differently. Tastes are subjective, but the staccato shot clearly has a lot less crema, hence it must extract differently.

PS the way I set up the staccato is very easy, and requires only a little bit of work. I split the dose in 2 parts, and grind them at different granularity, and tamp one on top of the other. Using a sifter would be too much work.

I've been thinking more about the physics of espresso, and why the staccato shot may be interesting.

As far as I can tell, the main issue is uneven extraction, and that can happen for multiple reasons, some potentially good and some potentially bad. A reason that tends to make terrible espresso is channeling, and you'll always have some amount of channeling. The reason for this is that the water resistance is not even in the puck, and water follows the path of less resistance. When the puck is dry, there's less resistance and that resistance increases as it gets wet. Then once you start having water pour thru, it reduces the solids inside the puck creating less resistance (note that this is inherently an unstable process -- if you get more water in one area then it becomes less resistant thru that, and amplifies the channel). During this process the resistance in the pack will vary considerably, and that creates channeling. For example, the resistance at the top of the puck higher than the resistance at the bottom. There are methods to ameliorate this.
1. lower maximum pressure -- this put an upper bound on the maximum vs minimum pressure, hence might reduce channeling
2. preinfusion -- this wets the puck before full pressure. The worst thing is when you have the wet top puck and dry bottom puck, ie, highest pressure differential. But with preinfusion, this is avoided somewhat. Even more, if you do blooming, the water will saturate the puck even more, with potentially less channeling vs say, a Slayer shot.
3. bottom paper filter -- this puts a lower bound on the minimum resistance at the bottom of the puck. If you can pre-wet the bottom of the puck a little before the top, this may give even better results, hence some people wet their bottom paper filter.
4. weber unifilter -- this creates more even pressure because it's straight vs regular baskets that are smaller at the bottom. Also, wider baskets will decrease the thickness of the puck, which also helps.

Nothing so far addresses the problem directly, like the staccato shot. By putting finer grinds on the bottom, you basically equalize the pressure inside the puck somewhat, in an orthogonal way to preinfusion or full pressure or flow control. Pressure or flow control will not get the effect of the staccato shot.

I've only see a couple of people experimenting with this; there's mostly 1 guy who did most of the experimentations I think. It'd be nice if more people experiment

My admittedly lazy emulation of the staccato technique:

1) Grind into a catch cup, waiting to the very end to purge out any fines onto the top of the grind.
1a) For extra coffee-nerd points, gently adjust the grind finer as it proceeds.
2) Tap tap tap the cup of grinds in the catch cup. This will subtly migrate fines upward.
3) Place wetted filter paper in the bottom of the basket and invert onto catch cup.
4) Quickly flip rightside-up, such that the grind in the catch cup falls like a piston into the basket.
5) Rake just the top of the grind to level, then tamp firmly.

So, more like a 'gradiente' than a staccato. The abrupt transitions in grind size may be more of an artifact of the grind sorting process than some sort of sciencey reason.