Pushing the front of your lid vertically upwards (from your brow) would probably lift it off your head, but pushing horizontally on it shouldn’t move it appreciably IMO, unless you have no perceptible brow, or the helmet’s designed to sit way high on your head (unlikely, given it’s role in life).
It’s crucial that your lid fits well all over your head, (FIT) and that it doesn’t move around much during an impact (FIT & CHIN STRAP TENSION). If it’s not snug against your bonce the polystyrene wont dissipate the force of the impact as it’s supposed to. Least that’s always been my understanding of it, same for motorcycle lids too. I’ve just survived a high speed OTB head-plant which rote off my Fox Flux. I’ve replaced it with another one as it was an excellent fit, and had proved itself in service! (thankfully).