Rubby allows the player to pass through triggers without ever entering them, or to enter triggers multiple times by never leaving them.
It utilizes the fact that during 0-tick Saveloading the player cannot interact with triggers.
In order to perform rubby, the player needs to be moving either towards a trigger, or towards outside of one, While the player's VPhysics Bbox has priority, and then executing 0-tick saveloads.
This is usually done by placing a flat physics object on the ground, then walking on top of it.
Usually, when the player 0-tick saveloads they cannot move, but they can if the VPhys Bbox has priority.
Leaving a trigger when 0-tick saveloading will cause the trigger to not register that the player has left it, and passing through one will prevent it from registering that the player has ever went through it.
Utilising Rubby to enter a trigger twice in Half-Life 2: Episode Two, tricking the game into thinking that Alyx is together with the player: https://www.youtube.com/watch?v=m4m5NcaO1ek