Yellowcard allows you to yellow card a user which shows a yellow card on their pawn (File:Yellowcard-pawn.png). It is meant to be used as a warning before a user is banned.
When you have the power you will get a new option named “Yellow” listed under "Kick" (see example 1).
This power does not allow you to yellow card equal or higher ranks. Without gcontrol assigned, the default rank is main owner.