It's a feeling. The "front" is very literal for us; the human body has a line going down the center for the center of mass and the "front" is forward that, like where the third eye (chakra-wise) would be. Whoever is fronting is "physically" in that space.
When there's a switch, it's usually someone scooting out and another person coming in, a pretty seamless exchange. Sometimes it's more violent; there's been times when Aether has tackled me to take the front to do whatever (and it gave me the sense of vertigo like I WAS tackled/knocked over).
We like the term "front" because it's helpful for describing where other people are. Like someone "co-fronting" would be in the passenger's seat (assuming the "front" is the driver's seat)(I'm American so driver's left, passenger's right), but someone who is not fronting but is "present"/"awake" would be in the "middle" or "back". A lot of how we understand our system is the relation of "physical space" to each other. But the front is the front-est you can be; anyone else can only be to your side or behind you.
I hope that makes sense! lol
-bird