Polyphase Commutator
I think you are referring to the symbol shown at the left of the structure: the arm that moves between different phases?  See slide 4 of this PDF.

The point the Polyphase representation is trying to get across is that we give different samples of the input to different Phases of the Polyphase filter.  Polyphase Filtering is a Digital Signal Processing approach, so your input would only be defined at discrete locations.  Between the samples, we do not know where the signal was (unless it is bandlimited...).  

So you can think of it as a memoryless switch.