My short answer: Nothing! I program rest into workouts because they are necessary. I have done all the workouts that I design for other people. I know when and where rest is necessary and when it’s not.
Sometimes people attend fitness classes at gyms or other studios and have a hard time adjusting to this concept of rest. They bounce around in place, do jumping jacks, etc. because they can’t stand holding still. In one of my classes, these people are the ones that did not go hard enough or heavy enough in the workout. They want the gold star for constant movement, not accuracy or intensity. They like to stay in that 55-70% zone of their max heart rate. It’s much more effective for overall conditioning to go above that zone when you’re supposed to, and take rest which allows you to hit this higher intensity repeatedly.
So if you are one of these people, up your weight or intensity next time instead of staying comfortable and constantly moving. This is when you will see change and actually progress in your fitness. Take the time to learn each movement to the fullest and never settle for half-assing a movement. At the end of the day, you only have yourself to answer to.