tl;dw: On older versions of Portal, setting mouse sensitivity to INF and using the strafe key to make mouse movement change position instead of look angle when the player is near a portal triggers a failsafe that sets the player coordinates to (0, 0, 0) if in the air or (0, 0, z) if on the ground. This can be used to speed up the completion of some maps for the in-bounds any% category.