If I understood it right, and I think I did, the sissa can move foward and then backward, or vice-versa.
I'm not very fond of it, sorry. So, I'd like you to think about quarter and half sissa.
quarter-sissa can make n(W)n(F), but only foward, another quarter-sissa can make, foward only, n(F)n(W).
The half-sissa can make either move of the quarte sissa above, i.e., n(W)n(F) or n(F)n(W).
Hugs.