As mentioned, it allows the feed side of the solar panel circuit to slowly drain back to the pool.
FWIW, I didn't like this idea when I put mine in, thinking that the small line could get blocked, and I don't like those type of fittings anyway. I put a three way valve above the solar pump (after a check valve) and I allow a small amount of the water from that pump to go straight back to the solar return line and into the pool. The valve allows me to fine-tune the flow to the solar panels, and the partially open side becomes the drain back to the pool. It's locked into position so no one will close it fully. It also gave me a backup pump for circulation.
A simple fix would be to re-pipe and plumb a 1/2" or 3/4" bridge with pvc and a simple ball valve in it. Close it down to the point where flow is correct for your panels, but never close it fully, so the system can drain back to the pool. I didn't do this because the risers to the roof are near the patio and I didn't want to risk a noisy valve. As it turned out, the 3-way valve is entirely quiet, I think because of the back pressure on the pool return side when operating, and the low pressure and flow rate when draining.
The cheap and nasty way that some installers use is to drill a small hole (1/4") in the clacker valve in the pump (if solar is on its own separate circuit). The hole allows drain-back through the pump, without enough flow to spin the pump backwards. I really didn't like this idea and wouldn't consider it myself. Some solar circ pumps come with the hole already in the clacker, so I suppose those manufacturers aren't worried about it, but it still seems wrong to me, FWIW.
