You've seen it a hundred times and it's the only reason they could make 10 Friday the 13th films. Every time you think Jason is finally, frozen, dismembered, sent to hell, and dead - oh dear god let him be gone - dead, he raises a hand or the rubble rolls around. It's as simple as that. There's no need for explanation or context, just a quick shot of a hint of his virility and the film series continues.