What pre-infusion techniques do you use, how, when, why? Taste impacts?
- stream or water bloom or both?
- how long?
- what pressure?
- for what types of coffees?
- taste impacts, in the cup?

Generally,

The lighter the roast, the harder to extract. So, grind finer, and pre-infuse longer. In most cases you can also get away with a higher brew water temperature as well.

Meticulous basket prep is important with these bitchy coffees that are harder to dial in or else you'll be hampered by channeling which will reduce the extraction yield leading to sour harsh espresso.

Specific, detailed recipes are fairly difficult to toss out blindly, not knowing the coffee, grinder, machine, etc. So the above is a general starting point that will get you heading in the right direction.

Here's my long form answer with video.

An Even MORE Considered Approach to E61 Flow Control (now with video)

Here's what I do with my Elizabeth, 95 degrees C, about 9 bar and dark roast.
Bloom preinfusion. Bloom pump on: 5 seconds. Total preinfusion time: 12 seconds. Total brew time: 34 seconds. I adjust the grind so coffee starts to drip at the end of the Bloom pump time and output volume is OK. After brew I purge system for 6 seconds and water is clear and particle free with use of a puck screen. The product is repeatable, good crema, and tastes good to me.

I do the pre-infusion not by time, but until the droplets appear at the bottom of the basket. This means that the coffee tablet has already moistened evenly over its entire height, and extraction can begin. On most machines with a group other than E61, pre-infusion is 5-10 seconds, which can be insufficient and even harmful: if only the upper part of the tablet is wet and the lower part is left without pre-infusion at all, we get vertical unevenness (the wet part will be extracted differently than the dry part).

I only use preinfusion as a way of preventing my machine from choking on a very finely ground light roast. Usually 5-10 seconds after it gets to around 2 bars will get me a few drops in the cup and then ramp to full pressure at that point.

I haven't done extensive test around taste, I've simply found that without preinfusion I will either choke the machine or get a really long shot that results in over extraction or I will get a bunch of chanelling that gives me bitter/sour tasting shot.

I don't use preinfusion as a way of tweaking taste, more to be able to produce a shot that flows evenly and results in a properly extracted puck.

Having used an e61 heat exchanger plumb in rotary pump the plumb in offered a line pressure preinfusion possibility I didn't experiment with much and got hit and miss channeling when I did. An e61 by design has a short preinfusion on a normal pull if I'm not mistaken, which did fine for me.

When using preinfusion I think waiting for full puck saturation is important as is the volume, grind and distribution consistency especially with fresh roasts. The bloom of a really fresh roast I think can cause a shower screen tamp which could effect the overall extraction. To ensure head space during extraction I got into the habit of under dosing with really fresh roasts. With under dosing I had to be careful with grind setting, distribution and puck prep.

Currently I'm using a Bezzera Strega lever. Preinfusion is built in. Let the vibe pump fill the extraction chamber and you can apparantly build up to an 11 bar preinfusion. Takes about 14 or so seconds. I still underdose with fresh roasts. Don't know if it's the ramp down lever spring pressure release during extraction, the preinfusion or both but the I find the extraction quality is better than the e61 rotary pump hx with consistently solid pucks and shots with greater aroma complexity.

To answer the question then, preinfusion in my experience was as much about the machine as the roast with levers having more control and benefit from preinfusion.

I only use preinfusion on my modified GS/3 AV for light and light-medium roasted coffees that are hard to extract.

I use a modified Slayer-type shot to pull singles as follows:

1. Grind very fine
2. Gear pump is set to speed that produces 8 BAR peak pressure (16% for my machine and a finely ground 7g puck)
3. Preinfuse at 3 ml/sec (needle valve in cold water circuit)
4. Peak pressure will be reached in 15-20 seconds and coffee will gradually cover bottom of basket and begin dripping off
5. When scale says 0.3ml has dripped into cup, bypass needle valve (results in about 6 ml/sec free flow rate)
6. Reduce pressure to about 6 BAR by slowly dialing back gear pump speed to about 10%
7. Reduce pressure as needed to maintain 0.3ml-0.5ml/sec flow rate, usually not below 4 BAR
8. Cut shot when target beverage weight / brew ratio is reached

Steps 5 and 8 are done automatically by an Android app connected via Bluetooth to an Acaia Lunar scale and Arduino microprocessor that's interfaced to the GS/3.