Collapsing back leg dilemma

I have always heard this term but there are many ways that people use it. Some people say that collapsing the back leg is putting to much weight on it too soon so that the body cannot get going toward home plate. While others have said that it is when your knee turns inward so you cannot get ample leg drive. Just curious if anyone has any insight to this, and what you can do to not collapse it.

Good question. Possibly getting on the ball of the foot too early?