What do you mean by "shoving your knees out?" Do you mean forward or laterally? It might be a bad cue if you mean forwards, just because that would put him in a spot where he'd put his weight on his toes. Typically, I tell people to push their hips back and bend at their knees the same time their hips "break." This gets them to keep their weight on their toes, and hopefully gets them to use their posterior chain a bit more.
I think he means pushing the knees away from each other, towards the sides. When seen from above, the knees should track directly over your feet. In a squat, your feet should be turned out ~30 degrees (depending on stance width), so your knees should likewise be pushed "out" 30 degrees. Initially, this needs to be done consciously and deliberately, especially as many beginners will have a tendency for their knees to do the opposite and cave in. The "push your knees out cue" fixes this and often allows for greater squat depth and posterior chain engagement.