Technique and timing for the underarm serve
From a technical point of view, you'll need to master your timing and preparation with the racket's head - an up-down-up motion and correct follow-through - if you're going to do a good underarm serve.
In competitions, the underarm serve is allowed but is rarely used as it's much less effective than the classic serve, which is more powerful and precise.
It's worth remembering that, if you want to play an underarm serve in a competition, there's no need for it to bounce before you hit it.
It's a valid way to start play and, when used wisely, can really catch your opponent off guard. Basically, it's something to try for fun… But remember to show your opponent some respect!