Johnny Chock is the patented method for flushing out your black water tank. Johnny Chock allows for continuous flow of water to your holding tank through the unblocked center of the chock. When Johnny Chock is in place there is no need to hold or wedge toilet flush valve with your foot, hands or a stick. Johnny Chock is also available with a toilet rinse tube to clean holding tanks. To use simply open the toilet flush valve with hand or foot and place Johnny Chock in toilet bowl where it will keep valve open until you remove it. Chock Only.