I do this this at the end of my workout - before cooldown and I've had so many people show me the results they've had because of it.
Start in Horse Stance (feet shoulder width apart, knees bent, back straight, guard up). Alternating jabs (for 1 minute - on the beat). For the next 30 seconds double time it. The last 30 seconds all out!
Go to uppercuts, alternating for 1 minute on the beat. Double time for 30 seconds. All out 30 seconds
Next, hooks. With hooks, make sure elbows are below shoulder level and don't go any faster than double time (it's too easy to injure that shoulder). Depending on how hard I worked them, I will either make the double time 30 seconds or 1 full minute.
Last, do backfist (in front of you on the bag). When you go to double time and all out, it will be like a speed bag. Elbows need to be at an angle toward the ground - not straight out or straight down.
Great toning for arms.