Double Inputs

Double Inputs are an exploit in HP1 and 2 that allow Harry to accelerate faster.

Method
In the pause menu for HP1 and 2 each action can be assigned to two different buttons including being able to move in different directions (by default up is bound to Up arrow key and W, down is bound to Down arrow key and D). By pressing both buttons at the same time, Harry will accelerate faster from a standstill. This can be increased even further in HP2 through the use of a game controller. Note that this only increases Harry's acceleration, not Harry's movement speed.

Uses
Double inputs are most often used just as general movement tech, rarely saving more than half a second. The only glitch or exploit that requires the use of double inputs is rictusempra skip in HP2 120fps Any%. By holding both back buttons during the first cutscene skip in the rictusempra challenge Harry can jump through the door in the spongify challenge before it fully closes, skipping the entire level in an any% run.