[smmnyc]

Off topic but I bet you could tell the difference between an espresso shot pulled after stirring the grounds to ensure even distribution vs one that hasn't been messed with. I'm not saying you can tell the difference between stirring with a toothpick vs stirring with tiny metal tines. You can actually observe a difference in how the water flows through the grounds, you can measure how much coffee has been extracted into the water with a refractometer, and you can do a blind taste test and identify which is which. If the grounds are not evenly distributed, you get channels where the water passes through under high pressure, and this over extracts those grounds making the shot taste extremely bitter, and other grinds are left untouched or under extracted. I was very skeptical of this until I observed it myself.

[mattgreenrocks]

Most people don’t realize that making espresso is quite complex due to seemingly minute factors like these. When you’re pushing water through grounds at very high pressure, there are a lot of things that have to line up to get a great shot.

[nicolaslem]

Not only a great shot but consistent shots, that's even harder. I really don't understand the gripe OP has against WDT because it is really about eliminating those minute variables so that the process is reproducible.

[ggreer]

I just tried this and could not tell the difference.

It seems like how one packed the grounds would matter more. If it’s slightly uneven, one side will get more water pushed through it.

[atchoo]

It is about improving the packing of grounds by breaking up the subtle clumping that occurs during grinding to reduce channeling where water finds an easy path so part of the puck is over extracted and other parts are under extracted.

If you already had perfectly even flow, it's not going to improve anything. That might be true with great equipment or a very forgiving bean. It might also be true if WDT isn't enough to fix your channelling alone so there is more left to fix. This leads you to watching slow-mos of your pulls and inspecting pucks :)