I want you to do an experiment.
If you don't strength train your upper body regularly I want you, for 3 days in a row, to annihilate your upper body with exercises. Do whatever it takes to get insanely sore. Endless pushups and pullups to failure would suffice. Then the 4th day, when you're so sore it feels wrong, I want you to trick. The point of this experiment is to teach you the importance that your upper body plays in tricking. What better way to learn this than to cripple your upper body and try to trick?
Watch Vellu's upper body:
There is something you should notice about the way Vellu tricks, he uses his upper body more efficiently than almost any other trickster. Watch him trick and look only at his upper body. He always puts his upper body in the best position at the right moment, and he is good at using it to commit to his spinning. Watch how he uses every trick he does in a way where momentum continues: the way he uses his upper body is a big part of that too.
What I'm trying to prove
I'm trying to prove that your upper body matters in tricking, that you should use it correctly, and a good first step in using it correctly is to be conscious of it and its importance when you trick. Again, what better way to learn that importance than to cripple your upper body and then try to trick?
Am I trying to convince you to start training your upper body? NO. Because if you notice the best tricksters are very slim, usually skeletal, and upper body training can easily lead to unnecessary mass accrual. Since my upper body size and strength has grown over the years through my strength training efforts, it has made all tricks much more difficult for me; I do it only because I prefer the muscular aesthetic.
What I'm trying to prove secondarily, is that if you are under the impression that strength training is a valuable tool for improving your own tricks, you shouldn't limit your efforts to only lower body strength training. If you're strength training for tricking, you must train your upper body too: because what you do with your upper body, and what it can do, makes a difference in your tricks! And there are many ways to train your upper body for improved strength and speed for the benefit of your tricks without accumulating excessive mass.
If you do want to train your upper body for tricking
If you do want to train your upper body in a way that would benefit your tricking without beefing you up, a decent recommendation would be to work on karate stuff. Hand combinations. Watch freestyle form competitors, they have very crisp, very tight control over their upper body in their tricking, because they train the hell out of their hand combinations and have fast arms.
Another decent recommendation would be to work on gymnastic exercises: Rings, P-bars, Hand balancing, etc. These things are like whole body exercises that happen to be upper body focused. For example, my legs even get good training when I work on rings, because the tension goes through the whole body during the movements. This whole body tension has awesome carry over to your tricking. Also, improvement in these gymnastic exercises requires you to keep your body weight in check: so if you are improving in these exercises, it means your tricks will be improving too to a certain extent.
How much? When?
How much and when to do upper body training is a matter of discussion that goes beyond the scope of this page. Also know this, that upper body training can temporarily interfere with tricking throughout a training period, (because a hard upper body session makes you sore and makes tricking harder duh) but as long as you allow recovery to occur and balance the training properly, the benefits will pronounce themselves progressively. Otherwise, if you're not interested in upper body training for tricking, at least discover the important role your upper body plays in your tricking by doing the experiment described at the beginning of this page. If you weren't already mindful of what your upperbody was doing in your tricks, you will know after that experiment. Thanks for reading, I hope this is enlightening!