Voltorb Flip
Voltorb Flip is a minigame of the Goldenrod and Celadon Game Corners in the Korean and Western releases of Pokémon HeartGold and SoulSilver. It replaces the traditional slot machines found in the Japanese versions and bears similarities to card flip, a Game Corner minigame exclusive to Pokémon Gold, Silver, and Crystal.
Among Japanese fans, it is known as ビリリダマめくり Biriridama Flip.
Mechanics
Gameplay
Voltorb Flip was described in a prerelease review as "a cross between Picross and Minesweeper".^{[1]} It features a fivebyfive grid of tiles, underneath of which are hidden numbers (multiplier cards which affect the player's coins) and Voltorb (which Selfdestruct).
Flipping a multiplier card will give the player that many coins on the first card, or multiply the total by the number flipped for all subsequent flips. Higher levels have more multiplier cards, therefore producing larger payouts. Flipping a Voltorb will cause the player to lose all coins earned during the current string of multipliers and end the game. If the player flipped fewer multiplier cards (including ×1 cards) than the current level number before the round ends, they drop down to the level equal to the number of multiplier cards flipped. For example, flipping 3 cards before losing will cause the player to regress to level 3. At any time, the player can Quit instead of flipping more cards to end the game and keep the coins earned during the current round (though the level drop down rule still applies); doing so counts as a loss in terms of the next round. The player wins and advances one level by finding all of the ×2 and ×3 multiplier cards. Winning or Quitting, as before, will earn the player Coins, which can be traded for prizes. Level 8 is achieved by winning five games in a row of any level having flipped eight or more multiplier cards in each of these five games (in the fifth game, you can Quit having flipped eight or more multiplier cards and still advance to level 8). Exiting the game will reset the player to level 1.
There is also a "Memo" mode where the player can set symbols with the Voltorb and the numbers 1 to 3 on every field which has not yet been flipped up in the current game. This can be used to indicate whether, for example, the player thinks a given tile has a Voltorb or not without actually flipping it over.
Once the player earns the maximum of 50,000 coins, they will be exited from the game and will be unable to play further until spending some coins.
In each level, a certain range of coins can be obtained, and a certain number of Voltorb can be present. Higher levels contain more 2x and 3x tiles (and therefore yield more coins), but also contain more Voltorbs:
Level  ×2s  ×3s  Voltorb  Coins 

1  
3  1  6  24  
0  3  6  27  
5  0  6  32  
2  2  6  36  
4  1  6  48  
2  
1  3  7  54  
6  0  7  64  
3  2  7  72  
0  4  7  81  
5  1  7  96  
3  
2  3  8  108  
7  0  8  128  
4  2  8  144  
1  4  8  162  
6  1  8  192  
4  
3  3  8  216  
0  5  8  243  
8  0  10  256  
5  2  10  288  
2  4  10  324  
5  
7  1  10  384  
4  3  10  432  
1  5  10  486  
9  0  10  512  
6  2  10  576  
6  
3  4  10  648  
0  6  10  729  
8  1  10  768  
5  3  10  864  
2  5  10  972  
7  
7  2  10  1152  
4  4  10  1296  
1  6  13  1458  
9  1  13  1536  
6  3  10  1728  
8  
0  7  10  2187  
8  2  10  2304  
5  4  10  2592  
2  6  10  2916  
7  3  10  3456 
Strategy
Voltorb Flip is luckbased in the fact that the placement of the ×1, ×2, ×3, and Voltorb cards are random. However, a strategy can be used to mitigate randomness since the game is gridbased and the player can see the number of Voltorb and the sum of the multiplier cards in each row and column along the bottom and right side.
If the total Voltorb in a row or column is 0, all cards in that row or column are "safe" to flip over.
One crucial insight to make is that only 2x and 3x multiplier cards must be flipped over to win the game. If a row or column contains only 1x or Voltorb cards (which can be proven using a certain requirement shown below), that row has nothing of value left in it, and can be left unturned to complete the game. These "dead" rows/columns should be marked appropriately so as to not take unnecessary risks. Rows or columns with point/Voltorb totals of 4/1, 3/2, 2/3, 1/4 or 0/5 can be automatically eliminated as there can only be a 1 or a Voltorb in them.
Dead row/column formula
Given:
 a = Total Voltorb in a row or column
 b = Sum of Multiplier Cards for a row or column
 c = Number of ×2 cards flipped
 d = Number of ×3 cards flipped
If a + b  5 > c + 2d, then the row is not yet "dead"  there is still at least one ×2 and/or ×3 card left to flip.
A second formula also may help to find dead rows and columns:
Given:
 a = Total Voltorb in a row or column
 b = Sum of Multiplier Cards for a row or column
 c = Number of Unflipped Cards in a row or column
 d = Sum of Flipped Cards in a row or column
If c  a = b  d, then the row/column is "dead", as all the remaining multiplier cards in that row must have a value of 1x. (To put it plainly, if the number of remaining nonVoltorb tiles equals the remaining point value of the unflipped cards in that row, it's a dud.)
A simpler calculation is also possible:
Given:
 a = Total Voltorb in a row or column + Sum of multipliers in the same row or column
Subtract the multiplier of each flipped card in that row/column by 1 and then subtract the result from the a value of the row or column; repeat with all the cards from that row or column cumulatively, until the result is 5.
Replacement
The replacement of slot machines with Voltorb Flip was likely due to stricter PEGI guidelines which required that any game deemed to "encourage or teach gambling" be classified at minimum 12^{[2]} at the time of release of HeartGold and SoulSilver, which had previously caused the replacement of slot machines with game machines in European versions of Pokémon Platinum.^{[3]} Furthermore, countries outside of Japan have developed an increasing distaste towards gambling and the exposure of it to minors since the release of Pokémon Gold and Silver, with these changing social values being the cause of PEGI's guideline revisions in the first place. The increasingly controversial nature of gambling in video games is the most likely cause of the Slot Machines' removal in all international releases of HeartGold and SoulSilver, including those outside of PAL territories.
The Voltorb Flip game is likely included to provide a fun alternative to the slot machines. The initial reviews of the game were positive, with the Official Nintendo Magazine stating the game is "awesome".^{[1]}
Voltorb Flip also does not require money to be played, with the player only losing the Coins they collected from the game.
Trivia
 This game seems to be similar to the cancelled game Pokémon Picross for the Game Boy Color (Not to be confused for released Pokemon Picross for the 3DS).
 The Voltorb can be thought of as a dramatic way of saying ×0, as it is roundshaped. The game ends at that point because after multiplying the payout by 0, no other multipliers will ever change the total payout to anything other than 0.
In other languages
