The Problem - Knees Caving in Uncontrolled while Squatting
If you ever find that your knees shoot inward while squatting, this is a big issue that should be corrected. Not only will you not be able to transfer force into the ground efficiently due to poor joint alignment, but you are also potentially setting yourself up for a big injury. In the bottom of the squat your knees should look aligned right over the top of your ankles. Your knees should be able to come forward but they are not meant to support weight when horizontally displaced relative to the ankles
The Fix - Ditch the "knees out" cue, think push out on heels and open up groin
The knees out cue has been used for years and you will commonly hear it used in many gyms today. Does it really make any sense though if you don't know much about squatting? Just think about hearing "knees out" if you were just starting out squatting. It creates a confusing situation for a new lifter who does not know anything about using their hips to create tightness
Instead, think about keeping pressure on the outside of your foot. Doing so will naturally bring about the desired effect of the "knees out" cue but you'll be engaging the hips and staying tight. You could also think about opening the groin as well. When you think about opening the groin rather than pushing the knees out it activates the hips to a larger degree.
If you don't have a coach or lifting partner to look at your lifts, simply film yourself from the front. If your knees are not lined up right on top of your ankles you have this issue. Use these cues to help you reach proper alignment and activate the hips. Some great exercises if you are still having trouble is wall squats and banded deep squat holds. Both will force you to learn to open up at the hips and the banded squat holds will make sure you activate the abduction of the hips to keep the band spread apart
If this is something that troubles you, send a vid my way at email@example.com and we'll get you squatting right!