Minecraft wiki entry for Development_versions, associated text: When powering either side of a repeater directly with another repeater, it will lock the current state of the other repeater until the signal to the powering repeater is turned off.
If a repeater that is on is locked, it will stay on.
If a repeater that is off is locked, it cannot output a signal.
When locked, the repeater's second torch will no longer be visible and a bar textured similarly to bedrock will appear.