The turtle is a defensive posture on all fours: knees under the hips, elbows tucked, chin down. It stops immediate back control and protects the neck but surrenders initiative — you cannot attack or pass from turtle, only survive.
The top player circles, looks for a seat-belt, and attempts to take the back or roll you to your back. Good turtling means keeping your elbows tight so they can't get the underhook, and your chin tucked so they can't access the neck.
From turtle the defender needs to act: roll to your back into guard, stand, or take side control. Staying in turtle indefinitely gives points away and burns energy.
For position details and escapes, see Turtle.