I am not sure about all the science stuff, but from my experiences, frequency and some volume add to a lifter building and adapting to more work, thus growing. Even when PL'er do lighter speed work a 2nd day per, they are getting more muscle work and frequency/volume in and it helps up the work capacity.
I am pretty much with Sean on this and feel hitting the major compounds exercises ie; training 90% of the body on differing days but 2x per, is near the top IMO, for natural lifters at least.