This is based on Specter Spelunker Shrinks. But instead of shrinking evenly, you can contract your width and height independant of each other. So it's sort-of like this.

Arrows: Move
Shift + Up: increase height
Shift + Down: decrease height
Shift + Right: increase width
Shift + Left: decrease width
Z: Hop

Your hop height is affected by your mass, which is roughly your height multiplied by your width.