First you need to swear to give up one classic aspect of the American game the static scrum half pop passing to running forwards, the is the Maginot line of rugby.
A dynamic ruck begins with presentation. The ball carrier must decide about 5 yards away from the tackle that he is going be tackled and prepare to take the tackle on his own terms. The photo below of BYU in blue vs Cal Poly San Luis Obispo is a perfect example of tacking the tacking on you own terms, BYU player has low body position, and is falling so he delivers the ball to his team even though he is double teamed. Keep you legs driving, keep you body low, maintain control of your momentum, go to ground on your terms. Don't try and pass, don't try and break the tackle. Going into a tackle is a fundamental skill every player must have.
Second key to a dynamic ruck is the forward plug. The next player into the ruck should see the ball and make and immediate decision to either ruck over or pick and plug the ball. If the ball is available, and there is a disfigured defense the first impulse should be to immediately pick the ball, run with a low body position into the path of least resistance. This should be a team wide mind set. Against a good opponent this opportunity might happen two or three times a game. If there is not space to pick and run the second player should set the ruck. This is more of a very high speed art than something I can articulate here. Below is an example of a Navy player executing the pick and plug technique in a dynamic ruck. He has a low body position, has the ball tucked in one arm and is quickly attacking a disfigured defense. In order to do this he must make a quick decision to attack.
The next key to dynamic rucks is the running scrum half. Instead of standing static dishing to forwards or back the scrum half reads the defense and runs to daylight ready to dish to forwards running at pace. Below is a prime example of Navy executing the dynamic scrum half move. He is running parallel to the goal line, and you can see the forward running at pace. this is extremely hard to defend.
The final dynamic ruck technique is the weakside attack. The idea is to keep rucking across the field with plugs, running scrum halves until you run out of field. Usually it is very hard to defend the weakside. Ideally you will attack all the way to the 5 meter line and put a man away with a few feet on the side lines. Trust me no team can keep defending the weakside all the way across the field. Below is CAL Navy 2005 game with Cal setting up the perfect weakside attack. Notice how Navy is forced to commit defenders to both sides of the ruck.
The last example is of Air Force vs Army in 2004 with AF scrum half reading the weakside attack. Army has no one defending, this is an easy long run if not a try if he just looked before automatically passing.
Keep it fast. Practice fast, think fast, be fast. Or as Coach Wooden would say "Be quick don't hurry."