Those are called kipping pull ups and are perfectly acceptable in crossfit, however some people especially heavier and just stating out can hurt their shoulders and elbows if they do them incorrectly so I kept it simple. Feel free to do them if you choose they add a more metabolic aspect to the workouts.
Generally 2 kip = 1 dead hang.
Norman, personally prefer dead hang as well.
Another thing you guys can try are is a modified L pull up, this is where you do your pull ups with your knees pulled up towards your chest. You will almost look like your are in a sitting position, this really engages the abs.