This is a feature I use almost all the time with my MPCs. It is very useful when recording drums or something on the first beat of the first bars without having to wait for the sequencer to loop one time.
Below is a quotation from the manual (MIDI sync mode)
“Pressing Run/Stop+Rec/Edit (which normally starts recording) will arm the record function, so that when a start message is received, recording will start with it.”
For example, in non-MIDI mode adding a shortcut like shift+Run/Stop+Rec/Edit or shift+Rec/Edit to arm recording would be very useful 
