Wow, those are really cool.
You have to think like the film.
That last one with the hand; one of the exposures is a hand silhouetted against a bright sky. On the film, this means that the sky has been severely exposed and all the silver will develop black and be affected. No new information can be recorded there.
But, where the silhouette is, this area hasn't been exposed nearly as much and the silver there is not very affected, and wouldn't develop up. Or in other words, there's "fresh silver" to make a new exposure with.
So, the 2nd exposure is of the girl with hands on face. This is recorded in the unadulterated silver left by the silhouette of the hand and has no effect on the severely exposed silver of the sky portion, so it looks like she's looking through a window in the shape of the hand.
All these types of multiple exposures work off this kind of thinking.
Thanks for posting!
(shoot, nathan beat me to it)