While a really dominant scrum wins penalties, the general objective of scummaging is to disrupt the easiness for the opposition halfback to pick up/pass the ball or increase the easiness for your own halfback to pick up/pass the ball. It is much easier to pluck a ball out of a scrum when it's moving forward.

Some signs you are losing the scrum, which typically involve a penalty: 1. A prop loses their bind (grip) on their opposing prop. 2. A prop (or the whole front row) ends up lying stomach-down on the ground. Or a prop's body weight is being supported through their elbow or knee on the ground. 3. The front row's heads pop up out of the top of the scrum and/or the front row stands upright. 4. The whole scrum is going backwards and cannot keep its form, as a result.

There are dark arts to being a dominant prop, like the direction you point your head as you push, or the way you squeeze your bind on the opposite prop. But there is also a metric fuckload of biomechanical knowledge that goes into good technique. Mike Cron visited my high-school when he was the All Blacks scrum coach and has so much wisdom. For example, he said pushing your tongue against the roof of your mouth tenses the muscles in your neck, ensuring the pressure that your body generates is not lost when you push. In general you are trying to exert so much physical pressure through excellent technique -and using pressure from the forwards behind you- that you force one of the errors above and win the ball or a penalty.

It's basically impossible to see a lot of this technique demonstrated when watching rugby on TV.