Because when your body moves through space, you have to contract every muscle in your body in order to maintain your form. (chestsculpting.com)
While my mind muscle connection improved on the basic barbell exercises, I couldn't contract my muscles as hard as I wanted. (getthisstrength.com)
Most guys do not fully contract the muscle on their lifts and instead use momentum to swing weights that are too heavy for them to handle. (shamelesspride.com)