Description: Drumbeat-like pumping regenerates the spin but the top never leaves the string.
Throw a trapeze. Shorten the string to about double the length of your forearm. While the top precesses pump it like a diabolo. Once per rotation go over the non-dominant arm to prevent the string from twisting. To do that: 1) lift throw hand above your head while also raising the other hand by bending the arm at the elbow; 2) guide the top between the forearm and your body. After the top clears the forearm, lower it to the center of the string and continue pumping.
Advice: Initiate the lifting when the tip is about to point towards you and clear the arm while the tip points towards the dominant side.
To clear the forearm you can guide the top by moving either hand or both hands simultaneously. However, I have more success moving the lower hand in an arc below the throw arm, staying ahead of the top tip.
To shorten the string while on a trapeze, wrap it a couple of times around one of the hands with a flip of the wrist, pop it in the air and grab the string at the correct length.
You cannot rush the trick or slow it down: the pumping has to be in sync with the precession. If you get behind and you need to raise the top too fast it will probably jump off the string. If you raise it too early you will lose spin waiting for the top to clear the forearm. After it loses the initial spin, just pump it two times, the second one including the lifting of the top as part of the pump.
Notes: You can also switch arms and go over the throw forearm while lifting the non-throw arm. The only problem is that the lifting does not regenerate and loses a little more spin.
You can even alternate between going over each arm for a different look (butter rolls). Finally, it is possible to go over both arms together but very difficult to continuously regenerate, as it takes longer going over and loses more spin.
History/Etymology: I invented and posted the trick in January 08. The trick derived from the drumbeat trick and so does the name: drum rolls (i.e., sustained sound on a drum), plus the fact that it "rolls" over the arm. A comment when the video was posted was "smooth like butter", so I propose the name butter rolls for the alternating-arms drum roll.