I couldn't tell you how many times I've seen non-yoga people do what I'm sure they think is a good stretch. They grab their foot and stand there for five seconds and no doubt feel that they've paid their due to the "stretch after working out" rule (see the inset picture).
If you want a real stretch, though, this one's better. You still grab the foot like before, but then you bring your other hand to the floor or to a yoga block. Or to a stack of books if you don't have a block. Or, if you're of the generation who doesn't know what a book is, stack ten of your most recent mobile phones, and that should do the trick. :-)
BENEFITS: The hamstring on the standing leg is definitely stretched. Ditto for the quadriceps on the leg whose foot you're holding. AVOID IF: Your bent knee hurts (avoid completely) or you IT band or sciatic nerve on the standing leg hurt (back off the intensity of the stretch till you don't feel discomfort.