I didn't read it very carefully but I guess this can be translated as, "If the particle is over here (it probably is), it isn't moving, but if it's over there

I didn't read it very carefully but I guess this can be translated as, "If the particle is over here (it probably is), it isn't moving, but if it's over there (very unlikely) it's moving very fast." Therefore most of the momentum expectation is coming from where the particle probably isn't.