The Princess/Swordsman spike is a specific tech which can be performed by Peach, Roy, and Marth. It is done by using a counter to setup an invisible ceiling glitch, causing players' next attack to knock the opponent at a different trajectory than usual. Specifically, if on stage it will send them straight slightly away and then straight down. If the player's attack knocks the opponent off-stage it will remove all vertical velocity of the knockback, but retain their horizontal knockback and hitstun. This turns strong attacks that have upwards knockback, into powerful spikes.
How to perform
To setup the ceiling glitch, the player needs to press L or R to shield (for the best effect the shield should be decently small), release shield by letting go of trigger or using jump, roll, or grab and Counter a grounded attack.
Then, the player should hit the opponent while they are airborne.
How it works
This likely works because Peach's Toad hitbox and Marth and Roy counter hitbox are very similar to the hitbox of a shield and are color-coded as the same type of hitbox as shields in debug mode. A standard invisible ceiling glitch is triggered by going airborne while still sliding back after a grounded attack hits an opponents shield, which can happen by either getting hit out of their attack on shield or for certain characters by walking or dropping through a platform after hitting an opponents shield with a specific attack. Normally if the player hit a counter it won't cause any of the shield push back required to trigger an invisible ceiling glitch. However, once the player have used their shield earlier on that stock when the player's opponents attacks the player counter, they will receive the same push back as they would have if they attacked the shield the player were previously holding up.
The player's counter will still be able to trigger the Princess/Swordsman spike until the player release a shield when it is incredibly light or the next time the player die. Also, moving around or using the counter do not undo the glitched shield + counter hybrid.
How the shield affects the spike
How light or strong the player's shield is before the player release it has an impact on how soon after the player counter will trigger the invisible ceiling glitch and the opponent will lose their vertical knockback. This is because how light the player shield is reduces the amount the player opponent is pushed back when they hit the player shield. So if the player release a lighter shield that is used for the player counter, they will lose their vertical momentum sooner. If the player make the player shield too light they may not have any sliding back momentum when the player counter hits, which means it will not trigger the invisible ceiling. If the player are too slow when the player release a hard shield, the game may register the light shield during the time the player spring pushed the player shoulder button back to its resting position. In this case, release the player trigger quickly or use an out of shield option to avoid the risk entirely. The stronger a move that hits the player counter is, the longer it takes for the player counter to trigger the invisible ceiling and the lighter the player can hold the player shield while still triggering the glitch.