The trick introduced in Pro Skater 2 that lets players link combos by balancing on two wheels is the Manual.
A manual is a wheeled balance maneuver in which the skater rides with either the front or rear wheels raised. In Tony Hawk's Pro Skater 2, manuals changed the scoring system because players could connect an aerial trick, a grind or another maneuver without touching all four wheels down. That made long, high-scoring lines possible.
The move also introduced a new technical challenge: players had to use directional controls to keep the skater balanced. A poorly balanced manual ends the combo, so successful players learned to correct their position while planning the next trick. The game even used balance feedback in some versions and ports.
Manuals are sometimes confused with reverts. A revert changes the skater's direction after landing and helps continue a combo, but it does not involve balancing on two wheels. Pro Skater 2 was also the first game in the series to include manuals and a level editor.