You can use the space bar to switch from one character to the other. The two characters slide on the ground and stop only when blocked by an obstacle. You must try to strategically place one creature in order to change the other one's path and clear the way for more collisions.
But watch out for the different types of enemies. They can act like a pinball bumper, reverse the commands, make the brown blobs invisible, make all the brown blobs reappear, block everything, and more.