Achieving the right speed OR the proper depth in CPR individually is much simpler than achieving both at the same time. Thus getting one of the two right can be accomplished through conscious control based on teacher instructions. Getting both right requires the motion to be done automatically which is more natural with real-time feedback. 

Nevertheless, the results also show that the training devices work best as an addition to human teaching. Trainees using a device for training after a teaching lesson improved their overall performance by almost 45% in comparison to trainees who trained first and later received teaching (overall enhancement only 24%).