From Hard Drop - Tetris Wiki
Yoshi is a simplistic puzzle video game for NES and Game Boy developed and published by Nintendo, similar to Hatris.
The playfield is 4 columns wide by 8 rows on NES or 7 rows on Game Boy. In each round, two blocks (Goomba, Blooper, Boo, Pirhana Plant, bottom egg half, or top egg half) fall. (At the end of each A-type level, three blocks fall instead of two.) Pairs of adjacent columns (1 and 2, 2 and 3, 3 and 4) can be swapped. If a falling block is in a swapped pair, it is swapped unless the stack in both columns are shorter than the height of the falling block. Two identical blocks "stacked" (aligned vertically) will disappear. A top egg with no bottom egg half below it disappears. A top egg half with a bottom egg half below it removes all blocks between the egg half, falls together, hatches, and disappears. A block landing out of bounds other than an egg top half ends the game.
There are no chains, and unlike in Hatris, there is no bonus for doing two clears with one pair of blocks.