After several very unscientific tests, I've found that I get a slightly better result on 3rd and 4th shots when I give my PF a cold rinse, as opposed to using the cold wet rag around the group head. When the PF is cool to the touch, I insert back into the group, grind (by hand about 25 seconds or so), remove the PF and basket, which by now is toasty warm, fill tamp, etc.
My 3rd shots are now as good as my 2nd (if I've done all the rest correctly! )
Thoughts?

A most excellent observation indeed! Whouldathunkit? In what time frame are these 3 or 4 shots taking place? And what time of day is this blast of caffeine happening? I'm sure they are not all sink shots!!! Kindof like the reverse of an HX machine. You need to come up with a name for this effect. HX is heat exchanger so what you've come up with is heat, let's see. HR, heat reducer? But R is not too sexy, not like X. I think for full modern effect you need the word nano somewhere in your process.
How ya doing? where have you been?

Sounds like thermal shock. Slightly cooling the PF is encouraging heat flow from the group? Bringing the temp into a sweetspot? Almost like an HX cooling flush. Inspired whatever is going on.

What I have learned is after a flush from the previous shot, to use a blind double basket and filled with cold water between shots. Don't lock the portafilter in tight and lift the lever high enough to suck up some of the water. Dry the screen and do the next shot. It is very effective...
Cooling the portafilter is cooling the espresso after it has passed through a hot grouphead and portafilter basket filled with coffee. Cooling the grouphead with a towel or using the technique I do daily gets the water somewhat cooler before it passes through the puck in the basket which is stainless steel and pretty resistant to thermal transfer from the surrounding portafilter. (Edited)

Eric and Doug....I've read here that some folks apply a cold wet towel to the group (especially on machines like the La Pavoni's, which tend to overheat somewhat after 3 shots or so.)
Certainly, On my cremina, I've done 3 shots within, say 4 minutes or so with good results. In fact I prefer the taste of the 2nd and 3rd shot, but for those more chocolate-y blends I prefer the temperature just a bit lower than I was getting on that 3rd pull. I like where my P-Stat is set, so I didn't want to go there.
As most here would say, the fruitier coffee's do better with slightly higher temps. Again, unscientific.
It's just so easy to slap that cold PF into that VERY hot group, especially after I've performed my "PF wiggle", which further heats the group.
It's amazing though, how quickly that group head "re-heats" itself, even with that cold PF in it. 30 seconds, and bam, ready to go. I guess brass really does conduct heat.

TUS172 wrote:What I have learned is to use a blind double basket and filled with cold water between shots. Don't lock the portafilter in tight and lift the lever high enough to suck up some of the water. Dry the screen and do the next shot. It is very effective...
Cooling the portafilter is cooling the espresso after it has passed through a hot grouphead and portafilter basket filled with coffee. Cooling the group head with a towel or using the technique I do daily gets the water somewhat cooler before it passes through the puck in the basket which is stainless steel and pretty resistant to thermal transfer from the surrounding portafilter.

What you say makes sense, EXCEPT that as the group is a conductor, wouldn't the presence of a cold PF help absorb some of the heat of the group? A heatsink, sort of? (Again, I'm clearly no scientist)
After all, when I remove the PF from the group 30 seconds later after grinding, the PF is more than warm, in fact fairly hot to the touch.
Also, I wasn't aware that with the PF NOT locked in, that raising the lever would allow water to be sucked up.
In fact, I always make sure not to lock in my PF while pulling a shot till the lever is just above parallel to AVOID the puck being disturbed by air suction. HMMM...food for thought.

Yes brass is an excellent conductor but consider that the portafilter makes its primary surface contact with the inner lip of a portafilter basket stainless steel (compared to brass is somewhat resistant to thermal transfer) and the portafilter basket then makes contact with a rubber gasket that is very resistant to thermal transfer. The only part of the Portafilter that actually touches the brass grouphead are the ears and just the bottom of them(very small surface area). So you are not cooling the grouphead you are essentially just slowly heating the portafilter with the excess heat radiating from the grouphead.
(Edit 12:38) Try using my technique (and I am sure others on this site have done similar things for years) You can actually do the same thing with a larger container of cold water by holding it up to and covering the bottom of the grouphead and pulling up on the lever (but not to the point where you are allowing any hot water in) Just work the lever up and down about a eighth to a quarter of the stroke. this moves the cold water in and out of the lower portion of the grouphead and also fully cools the dispersion screen. This is alot more effective but a bit more messy... water dripping off the end of the grouphead onto the drip tray... but nothing a quick mop with a towel doesn't solve. I don't do it because 90% of the time I do less than 3 doubles in a session with one machine.
And yes if you pull up on the lever with a glass of water or loosely locked portafilter with water in a blind filter, it does suck up the water. One of the advantages of having a lever is that you can do this. It is very effective at cooling the grouphead from where the heat radiates... the inside of the grouphead not the outside. Try it, I think you will see a marked difference.

Thanks for the explanation.

You are welcome! Glad I could be of assistance! Pull... Brother Lever owner... Pull!

As an aside: you can also use this technique to clean the dispersion screen with caffiza or other cleaning agents... great technique.

Richard Penney (espressme) has described (and I use) his "sponge in the PF" technique.
I'll look around for the thread in the Search area when I get some time. It's very cool.