Wait, knees go past toes?
Oh, no doubt. It works really well with some movements, but its not something that can be done across the board. If im deadlifting reps of 6, around rep 4 using the valsalva maneuver I find it incredibly difficult to maintain.
Knees pass toes, yes. I know about the arguments that the various associations have made against deep knee flexion, but biomechanical analysis shows that the greatest stress on the knee occurs at and above parallel. Going below parallel has not been shown to lead to pathology in healthy individuals. Look at the training of olympic weightlifters that squat heavy, frequently, and deep. They have surprisingly low rates of ACL tears and knee injuries in general. Of course there are many other factors (mobility, soft tissue health, structural balance, muscular activation, etc.) but deep squatting with knees passing the toes is not a problem (in healthy individuals) IMO.
http://general-stuff.38656.n3.nabble.com/file/n2911800/Squat_Depth.pdf
http://www.dentonisd.org/cms/lib/tx...erything_you_need_to_know_about_Squatting.pdf
Example of the form that I am talking about:
The front foot is elevated in the video, but is not necessary. It can be useful to elevate the front (or back) foot for variation.
As far as the valsalva, I think it's best reserved for maximal attempts (1-3RM). For deadlifts, lots of people like to essentially drop the bar and miss out on the negative. For people who train like that, the valsalva maneuver can be useful.