How does that work then? Do you just whack the tennis ball and then pull the string to get it back? Where is the ball when you whack it? Sitting on the ground or do you toss it up in the air?
I have it dangling from a string so with the length about a foot off the ground then aim to hit it away from me everytime it comes back.
The little variations in its line I found helped me move my feet better and it makes you move your hands through shots faster, was also useful for watching it onto the bat.
Every now and then you have to stop and start again, especially if you edge one which sends it spiralling around like a swing ball.
I'd change the height too to make it lower and higher and I practiced getting forward and back. Was a great tip I saw on here I think but it's a really nifty drill.