I just handled one at our local gun shop last night. The switch is only on the trap model made in the last couple of years. It is a collar that rotates on the magazine tube and it allows you to feed a shell directly into the chamber. The older ones you had to feed it into the mag tube and then rack it in. It is a pretty good feeling gun and for the money I don't think you could go wrong.
I have a regular BPS and you don't have to put the shell in the magazine tube. You move the pump until the "fingers" are toward the top fo the receiver and then slip the shell in to the chamber.. Don't know if it is the same for the trap model. It is a little wierd at first, but quickly becomes second nature.
If I have it set on repeat you can do the same with the trap BPS I have, you just move the fore arm slightly and the fingers move to the top of the receiver so you can slide a shell in, In the single mode the fingers stay at the top of the receiver at all times.
The BPS is a lot of shotgun for the money -- in both the trap version or standard. The mag stop is nice but those who shoot the standard version in the clay sports quickly learn how to operate it efficiently and courteously. Pretty good all-around shotgun in my experience.