Normal mode: Yellow Guy and two of his clones will appear at the edges of the screen and run toward the center. You need to right click the real Yellow Guy. You can tell which one is the real because he follows the following cycle where he appears: top - middle - bottom. If you don't press him, or press the wrong one, he'll kill you.

Hard mode: He can now make the sound cue, meaning that he will skip one or two steps in the cycle. If sound cue is low pitched, he will skip one step, if high pitched, he will skip two steps. If cycle is about to end, he can go to the next cycle and skip steps on the next cycle (for example, if you need to click on Yellow Guy on the bottom, and he skips one step, you will need to press on him on the middle next time).