Here is a video from a coaching seminar. It is in Russian, but summary is the following. There is a well known trainer for table tennis -- a wheel for practicing topspin shots. You hit a wheel with your racket trying to spin it, so you learn the proper contact with the ball. A coach from Belarus proposes to interleave training on the wheel with multi-ball. So first you make a shot on the wheel, making sure you had the proper contact, then the coach feeds you one ball so you immediately practice that contact on the ball. Then again you strike the wheel, then the ball. The point is that you immediately translate the contact on the wheel to striking the ball. And the wheel gives you immediate feedback on the type of contact you make. You can see a lot of demonstrations. He proposes to make wheels out of foam rubber so you can use your main racket without destroying the rubber.