In Super Smash Bros.
POW Blocks, based on their Super Mario Bros. 2 use, appear at intervals in certain locations on the Mushroom Kingdom stage. When attacked, it shakes the screen and deals 20% damage and considerable upwards knockback to every character currently touching the ground or a platform, often KO'ing characters over 100%.
 In Super Smash Bros. Brawl
The POW Block returns in Super Smash Bros. Brawl on the Mario Bros. stage, based on its use in the game of the same name. This time around, the block deals no damage, but causes grounded characters to bounce upwards (which interrupts charging smash attacks and the like) when struck.
It also lands a hit on every grounded enemy on the stage, and disappears in three hits (returning after a while).
The POW Block originates from Mario Bros., where it was used to strike all groundborne enemies on the stage at once.