TA, thumbaround, is a simpple and pretty easy trick. This can be done with the push of any finger. Generally its done with either the fingers 1 and 2.
First, decide which finger to push with,generally easier with finger 2, then place the pen with it resting on the finger after it. If, finger 2 was chosen you would rest it on finger 3. Then use your pushing finger to "push". The push is more of a snap, but just the motion of the snap. Keep your thumb straight and still. Don't worry about it falling because it won't unless you don't catch it.
The catch, it's fairly easy all you have to do is once it has gone around your thumb just open your palm and catch. The more difficult way is to catch it in between fingers 1 and 2.Kinda, hard to explain but you'll figure it out on your own, it comes naturally.
Tips:
-Don't push with too much force it will just make it harder for you to catch
-keep your thumb still and straight up the whole revolution around the thumb
-PRACTICE
https://www.youtube.com/watch?v=YzoF8zzKPMk&feature=channel_page