From Stunts Wiki
A controversial kind of shortcut that exploits a programming oversight in Stunts' penalty time system. On tracks where the road splits, it is possible to leave one of the paths and re-enter the track through any point of the other one without penalty time, provided at least one track element is crossed before the paths rejoin. Tracks with multiple paths usually either are designed in a way to prevent dual-way switching or explicitly exploit them to make alternative racing lines available.
Some canonical examples of dual-way switching. First, a basic tutorial on Siófok (USC 07/08). Then, a lap around X-change (SDR-RH 2007) featuring several way switches. For a more extreme example, you can also check the forbidden shortcut of ZCT127.