When White plays atari at , Black faces the choice of connecting at or connecting at a.
El Draco: I wonder if this could be called under the stones, since the normal under the stones always seems to incorporate an atari in the captured stones' space. Else you could also call playing at the vital point after a bulky five shape has been captured (not six stones as in the diagram, but just the bulky five) as playing under the stones. The definition on the under the stones page seems to apply to both cases, though.
Nevertheless, Black can get a ko out of it by playing as shown.