Well, it's not really so much a baseball game as it is a batting practice game. The mechanics are simple, the pitcher throws a ball, and you press the left mouse button to make your batter swing. Depending on how good you are at timing the press, you'll get either a home run, a clean swing, a strike, or even a foul ball.
There's also a timer at the bottom of the screen, so you can't take your time if you want to score a lot of points. Play the game straight from your browser, after the cut: