The Minecraft swimming glitch that allows a player to fit in 1x1 holes is a bug in the game that can be exploited to put a player into tight spaces. Here's how it works:

When a player enters water and starts swimming into a 1x1 hole, a glitch occurs where the swimming animation gets stuck and causing the player to look like its proning like in Ravenfield. This causes the player to fit into a 1x1 hole despite there not being any water.

To perform the glitch:

1. Find a 1x1 hole against a wall, preferably underwater.
2. Position yourself facing the wall and jump into the water.
3. Start swimming forward into the wall while staying aligned with the hole.
4. As you approach the hole, continue holding the swim forward button.

When done correctly, the player will glitch into the 1x1 hole, fitting in despite its small size. It allows for tight squeezes where players can access areas that would otherwise be inaccessible.