It involves setting a pin and the lowest possible Token at a specified station. This pin disables every player from playing a Loop over a station in the same map square as the so-called "broken" station. (If map squares aren't existent in the game, it affects every adjacent station and the adjacent stations of those stations.)
Furthermore, the Line Velocity on the affected stations is reduced to the lowest possible velocity. Four turns after the Break Point is set, the Line Velocity is Zero and stays so as long as the Break Point exists.
Depending on the map and the player's skill, setting a Break Point can either be disastrous or completely useless. Furthermore, every move involving a Break Point has serious drawbacks.