Here's a video where a coach talks about why he has his young players do self-toss hitting. I agree with him on its value.

Using wall traps and similar approaches is something coaches sometimes do to train hand contact, but they're of extremely limited value.

Here's a video with an interesting progression to help in young hitter training which could be useful if used properly.

Some coaches have a model of volleyball arm swing mechanics that doesn't match with what players really do in practice.