The "Tacker" has been developed to control the tack of the cruising spinnaker (asymmetric) by fastening it over the furled headsail and it allows you to use your conventional (symmetric) spinnaker like an asymmetric spinnaker from beam reaching to broad reaching without the need of the spinnaker pole.
"We think the TACKER is a terrific way for casual sailors to get more use out of their spinnakers" ... "The inexpensive TACKER allows you to fly a conventional spinnaker without a pole" -- Practical Sailor
Here is a link that shows a video clip of how the tacker works
