When I was growing up we used to throw baseball bats at 2nd base as a drill to learn to extend our arms and to keep our hands inside the ball. Then, when I was coaching I would have kids throw 1″ wooden dowels or pvc to accomplish the same thing.
In the late 1980’s I tried to hollow out a wooden bat so I could throw golf balls out of the end of the bat. It was an unsuccessful journey. Last year I was helping with some hitting lessons at Bob Clark’s CBI academy and a student, who is a good hitter, constantly hit the ball in the dirt (about 90% of the time). We wanted to show the student how to get more lift in their swing. On a Saturday, while at work, I picked-up a 1″ diameter piece of plastic, about the length of a baseball bat, and was swinging it to see what we could do to help her. I saw a piece of round pvc with a 1″ hole through it and it clicked, this could accomplish what I tried to do with the wooden bat. By the next lesson I had a 1″ wooden dowel the length of a bat with a stop on the end of it. It had a handle and between the handle and the end stop I had a 1.5 ” long pvc piece that would slide up and down the bat. When you swing the bat the glider(1.5″ pvc piece) would go up and hit the end piece and make a loud sound. Where you hear the sound would tell the hitter if they are casting or keeping their hands inside. To help the student get lift in their swing, I made a bat with no end piece. I put a 1″ hole through some whiffle balls so that when the student swung the bat the ball would come off and tell the student what would have to be done in order to get lift in their swing.