This is a closed chain single leg knee strength and muscle building exercise. Because your foot and body are supported by the machine there is less stabilising required by the rest of your body so you can focus strengthening and the tracking of your knee and hip. Start with the weights low and getting your technique correct before building up the weight.
Demonstration and Instructions
Slowly lower your body down using ONE leg. Go only as far as you are comfortable (above pain). Push back again through your heel aiming to 'push the plate away from you' and squeeze your glutes to make sure they perform this movement. Keep the pelvis level and squeeze/tighten the VMO but DON'T lock knee straight as you get to the end of your push out if you are hypermobile. Keep knee out over the middle of your foot throughout both parts of the movement.
Special Notes
Don't lower greater than 90 degrees if you get patello-femoral pain. Regression: Do an eccentric single leg press by pushing back with two legs and lowering with one leg. (You may also need to drop you weights down initially for this one.)