The Bug is somewhat difficult to reproduce. The event sheet contains a simple double jump code. The jump strength is changed if the jump is performed not on the ground. When pressing the jump control right before the transition from "on ground" to "falling" (by walking off an edge for example) it seems the air jump is performed however with the jump sustain which should only be performable when "on ground". Visually in the .cap file a single much larger jump can be performed jump is pressed right between the transition from "on ground" to not "on ground". (It might require a few tries to get the right timing to jump off a solid edge and perform the bug)
Jumping bug