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.