On Monday we went over a few basics of ball machines, specifically that you need to do a lot of homework on when, where and how often you'll be using one before plunking down a few paychecks to purchase it. Today, we'll get into the more technical aspects of ball machines - propulsion and oscillation. Propulsion is exactly what it sounds like - the way the machine propels balls toward you. There are two basic styles. The first is air-pressure propulsion, which simply means a puff of air shoots the ball through a tube and out into the big, bright world. This style is better for beginning players, because they are a lot more concerned about their own technique and motion than what kind of ball their opponent is hitting. The more advanced style is called spinning-wheel propulsion, which works just like a baseball pitching machine, using two wheels to fire the ball. This also lets the machine give you balls with top spin and back spin, simulating different styles of hitting. More advanced players favor this type of machine. Most machines use spinning-wheel propulsion, and the ones that use air pressure often come with an upgrade that simulates back spin and top spin. You should also check out to see the top speed the machine is capable of releasing balls at. If you're in a league where your opponents regularly hit shots 80 miles per hour, you don't want a machine that tops out at 60 miles per hour. Otherwise you'll feel like your opponents are John Isner and Andy Roddick next time you play a live opponent. Oscillation refers to the direction from which the ball is fired at you. It's all about angles, which by no great coincidence, is what a lot of tennis is also about. Every machine on the market can be set for random oscillation, meaning you don't know where the next ball is going. The higher-end models also feature the ability to program the oscillation, which means you can set it to shoot a high percentage of balls toward your weaker hand, or whatever shot you happen to be working on at the time. So, there are your basics on ball machines. Have a great weekend on the court and come back Monday for a new tip!