The game begins when two buses following the same route are within 1 or 2 stops range between each other.
The objective of the game is to get to the end of the route before the other bus. If a passenger wants to get off the bus or a passenger wants to get on the bus at a bus stop, the bus must stop at the bus stop and let them on/off.
It is this principle that brings the parkour element into play as both buses are overtaking each other as the route is followed up until the final stop (In the sense that the buses are "jumping over each other" as in parkour).
This can cause massive frustration for passengers both on and off the bus as Bus Parkour can slow down the journey and/or make the even later as they already are.
Dude 2: The bus drivers were playing fucking bus parkour
Dude 1: Fuck I hate it when that shit happens