In Modulus the direction of the belt is only determined by where you started dragging and where you ended. So if you start dragging from some input of a machine to some output of another machine, the belt will go in the wrong direction. In a game like Satisfactory, this is not possible. The belt will go in the direction the context allows (only from output to input). If you started dragging from output and try to connect to another output, it should visibly show the user that this is not possible.
(From brachy on discord)