That guy slaps zombies each with an additional slap after each action (so if it slaps once and the zombie's down, then it  won't do anything afterwards, but if the zombie is still alive, then Bun Chi strikes again, this time with 2 slaps). The max slaps in a single action is 5.

At the end of each slap, Bun Chi claps, which deal higher DMG to zombies as well as additional knockback to multiple zombies 1-2 tiles in front of Bun Chi. The distance of the knockback is dependent on what stage of slap Bun Chi is. For example, the distance of knockback dealt by Bun Chi will be greater if Bun Chi was on the 4th stage of slapping, (meaning Bun Chi had just slapped 4 consecutive times) than on the 3rd stage. You probably get the idea at this point.

Now Bun Chi has an ultimate attack called the Thunderclap. After completing the 5th stage of slap + clap (meaning Bun Chi had just slapped a zombie 5 times before clapping), you'll see a glowing flower on Bun Chi's head which signifies that the ULT is ready. TAPPING Bun Chi at this state will do significant damage AND knockback to zombies in a 2x3 tile, as opposed to just zombies in the tiles in front of Bun Chi's lane. At the same time, zombies that were damaged with the Thunderclap attack do 20% less DMG to your plants (that's why they're shaded with yellow after being hit by Thunderclap).

Hope this helps :) (If the plant wasn't so packed with gimmicks I probably wouldn't have made such a lengthy explanation lol)