Can someone explain the hand imprints?

I think its more symbolism than anything literal. The whole emphasis on chirality in the story might explain it as your hands are mirror images that cant be superposed onto one another. There may be -some- take on that the beach and reality/our world function in a similar fashion (mirror images)

It is explained by I believe Heart man in an early interview that you can read. He says that he believes the hand prints are there because the BTs are reaching out (with their hands) to the land of the living and that is why we see a hand print. They aren't physically walking or anything which is why there aren't footsteps. I guess it isn't a literal reason. It's not like bts have hands on their feet or anything.

Reading on Multiplayer.it, someone came up with a nice idea: the whole the abyss is represented as an upside-down world. Think about the large cetaceans that swim in reverse. Well if we were to move like that, basically we wouldn't use our feet... but our hands.

In a world upside down, where we would be pulled "upward" while upside down, we would move on the palms of our hands.

The BTs are reaching out into the world of the living from the other side. It's mostly symbolic, however when you get pulled into combat with a larger one you can see that one lighter shaded BT does, actually, come at you from above.

So some of them likely do move that way and we're just not seeing it because spooky shit, I dunno.

They can’t see so they use their hands.