Recently i went active again and looked up the current guides/forum entries of the community. Mentionable for this idea behind this glitch is the "Locked Room" Glitch found by @Wolfii & @AkagitsuneYuki .

I did research about how the game moves a player into another room.

1. The player interacts with a door.
2. The Game checks if the player can enter the door.
3. The screen fades out
4. Once the screen faded completely out, the player gets moved into another room.

Think about those 4 steps and some alertbell should start ringing. Need a hint? Where are you on Step 3?

Solution: While the screen fades out, we are still in the same room. This allows us to move in the room and interact with something before we get teleported into another room. Good examples for this can be found in the DLC, but there are also some cases in the Normal Run where this could safe some seconds.

The Glitch is used within my current DLC Speedrun. 1:05 is a good timestamp since i use the Glitch there 2 times in a row.