i dont think you should lock your knees... just went you cut, get your butt progressively low to the water. your knees should be at 90 degree angles right now. then at the last second, you stand tall (which basically means push down) so that you kind of ramp off the wake.
sort of the same technique, BUT with wakeboard the timing is more crucial! with a snowboard ramp, you still push off of it, but they are big. with wakeboarding, the wakes are small, and you have to have perfect timing.
just remember, you cant stand tall (push down) if you are already standing tall! make sure you let the boat start that progressive cut. at the end of your cut, close to the wake, you should be cuting hard, and your knees should be at like 90 degree angles. then push down at the top of the wake... without letting up on your edge! i almost think of it as pushing the boat away. if you try to push straight down, you will lose your edge.
last but not least, you handle has to be low for this whole thing. having your handle out straight wont help you