From the manual, people forget to read:
Push out solid
Only valid when the behavior is currently overlapping an object with the solid behavior. Automatically move the object until it is no longer overlapping the solid. This has no effect if the object is not currently overlapping a solid. The following techniques can be used:
Opposite angle reverses (or 'backtracks') the object from its current angle of motion until it is no longer overlapping.
Nearest moves the object in an eight-direction spiral out one pixel at a time until it is no longer overlapping. The aim is for the object to end up in the nearest free space, but since only eight directions are used it will be an approximation.
Up, down, left and right moves the object along a specific axis until it is no longer overlapping.