Well, I just read a paper on this for my project, beta. Basically, under drought, abscisic acid binds to those PYR/PYL receptors, which then blocks certain enzymes. This stops the guard cells from pumping in potassium, so they go limp and the stomata shut. It's like locking a door from the inside.
It's funny, my father calls me every week worried his own 'stomata' are closing—his shop, I mean—because of that big supermarket. I tell him his receptors just need the right signal, but he doesn't listen. In the lab, we see the clear pathway; in life, it's never so simple.
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