By doing this exercise you practice your technique and get your heart pumping at the same time.
How to: Set your timer for a predestined time, for example 3 minutes. Punch and beat the sack until the time runs out. Note that you can move from side to side while punching. Make sure you have got the punching technique right before doing this exercise since there could be a risk of hurting your shoulders or wrists.
Good luck ;)