I don't think they do, except in that they may give you a better sense of your knee position due to pushing against a material.
Do knee sleeves give a false sense stability?
Just used tommy kono knee bands for first time. If my form breaks down due to fatigue its normally my knees moving in towards each other. Recently when it happened it felt like less movement than usual.
Can/do they prevent valgus knee collapse directly? (I don't mean alot just a reduction of any amount of movement)
I don't think they do, except in that they may give you a better sense of your knee position due to pushing against a material.
I never noticed any support in that direction from my TKs, and TKs don't give as much support as a wrap in the first place. I wouldn't worry a bout it.
I use knee sleeves and have had a reduction in knee pain since using them on heavier sets. I highly doubt they affect the lift in any way, though. Sleeves aren't really rigid enough to correct form or increase rebound. I haven't really noticed a difference during the actual movement with or without.
What Rugby said makes sense, though that being the case then you'd have to understand that they aren't inherently correcting anything beyond giving you some synaptic feedback.