This is called the Zero Rest exercise. Basically, you pick 2 exercises, mainly calisthenic exercises, & you do 6-10 reps for each. You continue this for 15 minutes and then relax for 5 minutes before starting again and picking 2 different exercises. This is a killer for doing two exercises that workout the same part of the body, ex. shoulder raises and push ups. You're doing this in perfect form, not for the achievement of high reps, so you want to be doing these slowly and controlled. You should not be resting during the switching. I tried it today with squats and lunges and its quite challenging. You can keep adding more exercises if you want, ex. 10 squats, 5 burpees and 10 calve raises.
I got this from this site if anybody is interested for more info.