WASD or arrow keys to move. R to reset to the last checkpoint (just in case). The green balls are double-jump orbs. Note that you have to stop touching them ALTOGETHER before using it again—so you have to do some quick maneuvers, but you can use one multiple times, even if you're beneath it. Spikes and red things kill you. A flag is a checkpoint. Blue lines will connect you to the flag if you touch them. Also please note that if you die and come back to a checkpoint, the flag is white and will not turn green, but it still saves your checkpoint.
