Philip..
I don't know the psr-2100 but if it's like any of the other top quality units...and like squeak was trying to explain, You should be able to set a point to punch in at. This basically means you can accompany along prior to the actual recording location. When That point is reached, recording will automatically begin. but the location point, or whatever they may refer to it as, has to be set first.
Good Luck