When we talk about letting the knees unlock, we are referring to the phenomenon of the knees flexing or bending as you reach with your hips on the press. This is a no-no and is akin to letting your butt come up off of the bench when bench pressing - if it happens, the lift doesn’t count.
Consequently, when you reach forward with the hips to initiate the press, be sure to keep your knees straight - keep them tight, if you will - and with this in mind, here are a few cues to help you with this task:
“Straight knees” or “Straight legs”
This is the simple approach, and if it works, go with it. Merely remind yourself to keep your knees straight as you reach with your hips.
“Flex the quads”
Before initiating the hips movement, tell yourself to flex your quads - those muscles on the front of your thighs that are responsible for straightening your knees (and thus keeping them straight as well). Flex them so hard that you almost get a quad cramp.
“Lift your kneecaps” or “Pull your kneecaps up”
Before starting the press, cue yourself to pull your kneecaps up. If you’re not sure what this looks like, stand up tall and look down at your leg when it’s straight (your knee should be visible, so shorts or rolled up pants are necessary). Now, flex your quad, and as you do, you’ll see that your kneecap actually goes up an inch or so. Check out the video above for a demonstration of this.