It refers to clipping the leash to the TOP of the leash ring on our harness and to a regular collar simultaneously. Your leash snap holds both the harness ring and the ring on the collar together.
The chest strap on the harness is designed to pivot up to make it easy to reach a collar. We suggest loosening the collar if it has been adjusted tight for use alone with a leash.
After regular use, the collar-harness leash attachment can be phased out. However, the collar-harness is recommended in high stimulus environments.