Yes, those are Yakima stackers with the anchor/arrowhead shape on the end. I have nothing but good things to say about them. There are a few steps to using the stackers properly. First, get the distance between the bars correct to support the boat in front of/behind the cockpit. This is the stiffest part of the hull. Then lay the boats on the rack against the stacker posts. Run a strap around the stacker post (under the arrowhead), around the hull of the boat, and finally around the roofrack crossbar (as close as possible to the boat) in a big loop. Try to thread at least one strap through a solid point on the hull. As you tighten the straps, you may notice that the stacker posts tend to rotate towards either end of the kayak. This is just the rack adjusting itself. The stackers will reach an equilibrium position where any movement will make everything tighter (local minimum of strap potential, if that makes any sense). This is what you're going for. The Yakima instructions are well illustrated:
http://www.discountramps.com/marineImages/yakima-kayak-stacker-instructions.pdf
WHATEVER YOU DO, NEVER USE BUNGEES!!! A simple cam-buckle strap is THE ONLY tool for the job. Stay away from ratchet straps, too.