When having Swift Sneak equipped on pants, if you sneak backwards while holding "Place Block", it will frequently double place and place a block below the level you are on, basically clipping through the block you're standing on to place it.

This allows the player to do something that has never been possible before, which is to build a staircase down starting from the top.

Video: https://photos.app.goo.gl/kwbxiYNyNfkCCX4q6