I usually recommend using pressureless balls with ball machines for several reasons, but there is no specific requirement that you do so. One of the major reasons I recommend using pressureless balls is that they will provide a nice, consistent feed so you can really groove your strokes. Just using old tennis balls will work, but some will be more pressurized than others and you won't get a consistent feed. There is a downside to pressureless balls, though. Since they don't have an air-filled core, pressureless tennis balls are heavier than normal tennis balls and can make your arm a little tender if you use them a lot.
Most pressureless tennis balls are pretty similar and consist of a solid rubber core instead of a pressurized, air filled core. Tretorn Micro X balls are slightly different, as they are filled with thousands of air-filled micro cells.
Answered on 6/18/2012 by Ryan from Holabird Sports