Bounce is a common glitch in THPS series, causing skater to instantly bounce off the specific level areas in the opposite direction keeping the same speed.
It's the engine's task to choose the action to perform when game object reaches edge of the map facing the void. For example the most common case with holes in game level causes game character (human, car, etc.) to fall through the floor or fall off the edge of the map into the void which will either kill him or will fall endlessly in the worst case scenario.
In THPS1 and THPS2 the edge of the map acts like a wall. If the engine detects the void below skater, he is moved back to the previous location, so it doesn't affect gameplay in any way. However in THPS3 things have changed, now skater's speed wasn't affected, but his direction changed to the opposite when he faced the void, causing the bounce effect. Each Neversoft's PC game, but THPS2 allow this glitch.
Since it appears in several games, bounce usage is considered as technique by players. In most cases it allows to gain additional spin or change combo direction without loosing speed or balance. It is widely used in THPS3 and THUG1.