Wings & Block Duplication
Collecting Yoshi wings while riding Yoshi will complete the normal exit of the current level, as well as permanently changing the current Yoshi to a blue one. There are only 3 levels in the game that are intended to have wings (Cheese Bridge Area, Valley of Bowser 2, and Way Cool), but it is possible to get wings in other levels through glitches.
Glitched wings are a consequence of several glitches and quirks, the most important being block duplication. In speedruns, they can be used to finish several levels more quickly than intended methods, both due to less travel required, and because finishing a level through Yoshi wings skips the end-of-level fanfare that normally plays when Mario triggers the goal. Block duplication is crucial to getting wings, and since many different methods for duplicating blocks exist, it is discussed in more detail below.
Block Contents and X Position
The contents of some types of ? blocks depends on their horizontal position. In particular, the same block type may contain a key, Yoshi wings, a p-balloon, or a green shell, depending on its x position. If a level has a p-balloon or key block, it is possible to duplicate those blocks respectively to the left or right. The resulting block will hold Yoshi wings instead of the original contents.
Dragon Coin Glitch
Dragon Coins are two tiles large, and there is unique code that handles them for this reason (compared with regular coins). If a solid block is duplicated over one of the two tiles of the coin, the other tile will remain. When the remaining half of the dragon coin is collected, the solid block will then be replaced by an invisible ? block (which will become visible if it is re-scrolled onto screen). Coincidentally, this ? block is of the same type that can hold wings, depending on x position. If the x position is correct, Wings will spawn from the block. If it is wrong and a p-balloon or key would be contained in the block, one can now duplicate this invisible block (as above) to make wings.
Because natural p-balloon/key blocks are relatively rare in SMW, this glitch drastically increases the number of levels where getting unintended Yoshi wings is possible.
All block duping will require moving with 0 speed. Keep in mind that even moving with a speed of 1 while trying to dupe will fail.
Line up against the desired block to dupe and kick the shell upwards. By floating down with cape Mario for this dupe, it becomes a 2 frame window. There is no luck involved at all with a vertical dupe. The best example of vertical duping is Gnarly Wings.
Video: Gnarly - Cape (Wings, Quad Dupe) Source: Sten
This dupe executes the exact same way as the vertical dupe, but aims to duplicate the block to the side. Shells and other objects, when grabbed, store the momentum they had when picked up (just like in the buzzy jump in Vanilla Dome 1). To dupe leftward, grab a shell with rightward momentum stored and vice versa to dupe rightward. Once again, floating down with a cape makes this dupe a 2 frame window, but this time there is a 50% chance to dupe vertically and a 50% chance to dupe diagonally. However, if the dupe is attempted one frame too late, it instead becomes a 50% chance to dupe horizontally and a 50% chance of hitting the block. The most common example of the momentum dupe is Forest of Illusion 1 wings.
Video: Forest of Illusion 1 Normal Exit - All Castles (Wings) Source: Sten
This dupe isn't used in regular speedruns, but is important nonetheless. First, stand in a 3 pixel window to the side of a block (not against it). If using cape Mario, float down and kick the shell upwards and if using small Mario, simply jump straight up then kick the shell up. This gives an 8 frame window for cape and a 4 frame window for small Mario. This gives a 50% chance of duping horizontally and a 50% chance of hitting the block. However, if Mario is one pixel too high, the block will dupe diagonally instead of horizontally.
This dupe is an offshoot of the horizontal dupe used for Vanilla Dome 1 wings. By attempting a horizontal dupe against a wall of blocks instead of against a single block, the 50% chance of hitting the block gets entirely mitigated, leaving only the 50% chance of horizontally duping the block. The frame window for the dupe also becomes much larger.
Video: Vanilla Dome 1 Normal Exit - Cape, Yoshi (Fast Wings) Source: Lui
This dupe is another offshoot of the horizontal dupe but isn't used in regular speedruns. If a block is one block above the ground, simply standing in the correct spot and kicking the shell upwards will work. Once again, this has a 50% chance of turning the block and a 50% chance of duping horizontally.
By manipulating the subpixel of a shell, the floor dupe can be attempted with more than a one block gap. This dupe is required for wings in Yoshi's Island 3 but nowhere else. Hit the red koopa out of his shell and grab his shell, then drop his shell on the ground and grab it after it stops moving 5 times. After doing this, stand in the right spot and kick the shell up. Once again, this will give a 50% chance of duping horizontally and 50% chance of hitting the block.
By lodging an object between two blocks, one can be duped every other frame as each block tries to push the object out into one another. This dupe is used for Tubular wings.
Video: Tubular - Cape, Yoshi (Faster Wings) Source: SilverStar6609
Blocks do not necessarily need to be next to each other to produce a block factory. A "corner dupe" will produce a factory of blocks by kicking an object into a lower, solid block that has a corner touching a duplicatable block. Unlike the regular block factory that takes place against a wall, this will require being further away from the lower, solid block, as if to do a horizontal dupe but slightly higher.
This dupe is used for Valley of Bowser 1 wings only. By spinning two turn blocks one after another, then waiting for just the right moment, then kicking the throw block up in the right spot and moving out of the way, the turn block will bounce up all on its own, imitating being kicked up into the block from the other side. This performs a "wall dupe" from the other side of the block.