From the Super Mario Wiki
The POW Block (standing for Power Block according to The Super Mario Bros. Super Show!) is a common block in the Mario series. In all games they appear in, their effect when thrown to the ground or hit in the air will cause a large tremor on the ground, causing all enemies on screen to get hurt. POW Blocks can be carried and thrown in most games, and some are required for a 100% game completion. Most POW blocks are a metallic blue, but some POW blocks are red.
 Mario Bros.
The POW Block is an item that appears in Mario Bros. (the arcade version and its ports; not the Game & Watch one). When hit, it damages all enemies in the screen, but only those that were on the ground. It can be hit a certain number of times before it disappears. Each time it gets hit, it will flatten up. In some ports of the game, the POW Block can be picked up. However, throwing the picked up POW block will result in it completely disappearing regardless of how many times it was hit before.
 Super Mario series
 Super Mario Bros. 2
A giant version of the POW Block shows up in Super Mario Advance. A Giant POW Block can bounce around the floor, causing multiple tremors, before it disappears.
 New Super Mario Bros. Wii
The POW Block returns in New Super Mario Bros. Wii. Like in Super Mario Bros. 2, it can be picked up and carried, and once thrown, it eliminates all the enemies onscreen and also drops all floating coins. The POW Block is also essential in order to collect Star Coins found hidden in some levels.
 Trading Card
Description: Here's a blast from the past! Hit a POW Block and all coins will fall to the ground for easy pick up. Use this time wisely to collect all the coins you can. Remember that extra coins mean extra lives, so POW Blocks are money in the bank! They can also be really handy in shaking out hidden Star Coins.
 New Super Mario Bros. 2
POW Blocks return New Super Mario Bros. 2, but in a red variation appearing similar to those in Super Mario Bros. 2. Unlike the blue type, these POW Blocks cannot be picked up and float in the air like other blocks, so they must be activated by hitting them in some manner. In addition, they start a chain reaction with one another when activated, and are apparently unable to defeat on-screen enemies out of its range or drop midair coins, but they can be used to get coins out of Brick Blocks and Question Blocks, as well as smashing any normal Brick Blocks.
 New Super Mario Bros. U
 Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
In Super Mario World 2: Yoshi's Island, if Yoshi hits a POW Block, all nearby enemies turn into stars. It only appears in The Cave of the Lakitus, but appears twice in that one level. They can be hit thrice before it disappears. POW Blocks were most commonly won in Bonus Challenges, where they could be used anywhere afterward, being called Anywhere POWs.
 Paper Mario series
In the Paper Mario series, the POW Block is a portable item that does two damage to all enemies (but much more in Super Paper Mario if the remote had been shaken with each corresponding prompt). It penetrates defense and makes enemies with shells (such as Koopa Troopas) flip over.
In Super Paper Mario they are used as a portable item, that when active damages all the enemies currently on screen. In this game, the POW Block may also be used as a recipe ingredient; however, it can only be cooked on its own into a Megaton Dinner.
In Paper Mario: Sticker Star, POW Blocks appear as stickers, taking their design from how they have commonly appeared since Mario Bros.. When used, a POW block appears, which Mario hits twice to inflict damage to enemies, and occasionally inflict the Crumpled status. Unlike the previous iterations, however, the POW block only affects foes on the ground. The sticker also has a Shiny and Flashy POW Block variant, which deal more damage.
 Mario Tennis series
 Mario Power Tennis
POW Blocks make a small appearance in Mario Power Tennis, on the Mario Classic Court. A POW Block moves back and forth across the net. If a player hits the ball through it, all enemies on that player's side of the court become stunned, allowing the player to defeat them by running into them (This counts as one of two hits needed to stun Sidesteppers; Freezies are defeated instantly). POW Blocks cannot hit Fighter Flies when they are airborne though, but they could be defeated if the player hits the POW block at the correct time. After a certain number of hits, it will disappear. After a while, it will reappear in the course. Stunned enemies who got hit by the POW block will resume walking.
 Mario Tennis Open
 Mario Kart series
 Mario Kart Double Dash!!
 Mario Kart Arcade GP 2
 Mario Kart Wii
The POW Block is a new item in Mario Kart Wii. When dispatched by a player, all racers ahead of the user finds POW Blocks above their karts. The POW Block makes players spin out in a similar way to that caused by a Banana Peel, except that they lose all of their current items. Players won't be affected by the POW Block if the third strike goes off while they are in mid-air. Players can also avoid it by starting a wheelie, or shaking their controller (like doing a jump trick) right when the third strike goes off. By doing this, they still spin out and lose items, but retain their momentum. It is also possible to avoid it by doing a hop right at the third strike but requires perfect timing. When players have a Star, Mega Mushroom, or Bullet Bill activated, they won't get affected by the POW block. The letters, "POW", can be seen as a license plate of all of the cars in Moonview Highway.
 Super Smash Bros. series
 Super Smash Bros.
POW Blocks appeared as part of the hidden stage Mushroom Kingdom in Super Smash Bros. If a fighter hit the POW Block, all other fighters touching the ground would be damaged and fly to the sky. The POW block is a powerful stage hazard, and it appears throughout the stage randomly. After being hit, the POW block will reappear somewhere else on the stage.
 Super Smash Bros. Brawl
The POW Block makes an appearance on the Super Smash Bros. Brawl stage Mario Bros. where it can be used to paralyze all enemies on the screen, allowing them to be picked up and thrown at other players. After it disappears as a result of being hit a couple of times, it will reappear again in the same spot in the stage.
 Mario Super Sluggers
POW Blocks also appear in Mario Super Sluggers in the form as POW-Balls as an item. The item is an unlockable item that stuns fielding players for about four seconds. They are unlocked when a certain Treasure Chest is opened in Mario Stadium. If the POW-ball itself hit the outfielder, then only the outfielder gets knocked out and the POW-ball disappears, with no tremor effect on the ground.
In this game, POW Blocks are used to erase all the bits in the bottom part, once it is used, it will consume one restore space.
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