How would the left and right surround speakers on a 5.1 system make sound sound like its coming from behind ?

If your system doesn't have a 8 track, cassette and turntable throw it out

Assuming you're talking about a soundbar, if it's properly placed, the room is shaped the right way, and the listener/viewer is in the right spot, a very good sound bar will simulate surround, by bouncing left and right signals off the walls and back behind where the viewer/listener would sit.

Cheaper soundbars will not work.

You can spend a little bit more, and get individual wireless speakers and place them behind you.

virtual surround sound

It'll be your perception of it.

The placement of speakers is the clever bit.

psychoacoustics