As others have said, the server will bring a scanner to the table at the end of the meal to scan your band. If you're on a Dining Plan, they will also scan it before the meal to see which plan you're on (in case you're a guest who doesn't quite understand what you get with the DDP).
However, if you're using a DDP (perhaps also with CC purchase, I'm not sure), you will have to write your tip on the receipt and give it back to the server for them to finalize the transaction. Whether I tip well (which is most cases), or low, I have an aversion to the server seeing the tip before I leave. So after we discovered this "glitch" in March, we planned ahead and took cash on our August trip. I just wrote CASH on the tip line each time, and left cash in the final check folder.
Just a heads up.