In the video I slide under a block into a 1x1 gap. I accidentally came back out very quickly so it looks like just a glitch, but I assure you, you can walk around under there. To replicate, create a 1x1 gap at leg height, and then sprint at the block above the gap. If you let go of all movement keys just before you hit the block you will "slide" into the 1x1 gap. It is quite inconsistent, but I found having a shorter run up to make it more consistent. It has happened under other circumstances such as crouching next to the gap, but this method seemed to be the most consistent for replication.