If I understand your question, you may be confusing a tack downhaul line
with a guy. With a Tacker in place, there essentially is no guy.

No confusion. Just never seen or used the thing.

Rather, a
line from the tack of the chute keeps the Tacker/tack from rising up the
forestay. It feeds through a block at the base of the forestay chainplate
and leads aft to the cockpit. Gybing the chute requires releasing the sheet
and allowing the chute to gybe outboard of the forestay while bringing the
sheet around the forestay manually. Or if the air is light enough, one
could walk the sheet and sail inboard of the forestay to "gybe." The latter
will wrap the chute halyard around the forestay, though. I've got double
sheets on my spinnaker, so I release the old leeward sheet and allow the
sail to gybe outboard of the forestay while sheeting in on the new sheet.

Now, I am confused. All this sounds like it applies to an assy and not a
symmetrical spin. I thought these tacker things were for symmetrical
spinnakers, because assys already work like a tacker - i.e., sail generally
secured to bow of boat. Some fly assys with poles, but that defeats the
purpose for cruisers. I thought the tacker was to allow you to fly a
symetrical spinnaker on a reach without the pole. If you have an assymetrical
spinnaker, you probably have a tack arrangment and why would you need a jury
rig thing like a Tacker?