You have to place someone on the Disabled List for at least 15 days. It's to prevent teams, I suppose, from putting starting pitchers on the DL in between their starts and juggling their rosters and doing all sorts of nonsense.
But, you can start that 15 days retroactively to the day after the last time that they played. For example, Chase Utley didn't play today but played yesterday. If the Phillies decided tomorrow that he was hurting and they wanted to put him on the DL but only really needed 7-10 days to heal up, they could tomorrow say that he was on the DL starting today. And then he could come off it 15 days from yesterday. That's all.
Heh. Never mind. They just used Utley as a Pinch Hitter. But you get the point, anyways.
Long story short, you can't be on the DL the same day that you played. But you can decide some point later on that the days that you sat out, you were actually on it. I guess it has some merit insofar as it allows teams to take a, "wait-and-see" attitude that they don't have to decide the first day someone's wrist is hurting if they want to put him on the DL or see how he feels a few days later.