Princess / Swordsman spike

The Princess/Swordsman spike is a Peach, Roy, and Marth specific tech. It is done by using a counter to setup an invisible ceiling glitch, causing your next attack to knock your opponent at a different trajectory than usual. Specifically, if on stage it will send them straight slightly away and then straight down. If your 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, you need to press L or R to shield (for the best effect your shield should be decently small), release shield by letting go of trigger or using jump, roll, or grab and counter a grounded attack. Then you should hit your 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 you hit a counter it won't cause any of the shield push back required to trigger an invisible ceiling glitch. However, once you have used your shield earlier on that stock when your opponents attacks your counter, they will receive the same push back as they would have if they attacked the shield you were previously holding up.

Your counter will still be able to trigger the Princess/Swordsman spike until you release a shield when it is incredibly light or the next time you 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 your shield is before you release it has an impact on how soon after your counter will trigger the invisible ceiling glitch and the opponent will lose their vertical knockback. This is because how light your shield is reduces the amount your opponent is pushed back when they hit your shield. So if you release a lighter shield that is used for your counter, they will lose their vertical momentum sooner. If you make your shield too light they may not have any sliding back momentum when your counter hits, which means it will not trigger the invisible ceiling. If you are too slow when you release a hard shield, the game may register the light shield during the time your spring pushed your shoulder button back to its resting position. In this case, release your trigger quickly or use an out of shield option to avoid the risk entirely. The stronger a move that hits your counter is, the longer it takes for your counter to trigger the invisible ceiling and the lighter you can hold your shield while still triggering the glitch.