"Max Knee" is the max forward position, as measured horizontally from an imaginary vertical line bisecting the BB in MM, your knee will reach when your leg has the optimal 30 degree angle at the lowest part of the pedal revolution, when properly fitted to the bike.
Going beyond this point results in possible injury to tendons and loss of power. Conversely, not reaching your Max Knee limits your total power output. If you are properly fit on a bike, Max Knee will inherently be attained.
"I get Max Knee from my riding position now that I adjusted my saddle 5mm aft. it feels awesome, and I think it increased blood flow to my junk."
"Max Knee is great, no longer do I have incredible knee pain from riding a 50cm frame when my inseam is 34 just so I could have 'max stem showing' while doing bar spins in front of Johnny Brendas."