is oshi-tsubushi, literally "squashing push", a Japanese term (also used in English, but in English it is called internal liberty shortage by Robert Jasiek) for a life and death tesuji. The everyday translations, crush, squash, and smash, are also occasionally seen.
Under rulesets which allow self-capture, is possible; the three White stones are then immediately removed. If White can play at the point it would kill the black stones. now makes life, and so because Black must respond to attain it, the self-capturing move can be used as a ko threat.
If Black misses the oshi-tsubushi and instead takes the ko with , White plays under at and Black will die. Taking with is useless; White throws in with 4 at and Black only has a false eye here. (If instead of White mistakenly treats this as a ko fight, making a ko threat and then retaking the ko, Black gets a second chance at oshi-tsubushi.)
Black lives with the sacrifice and oshi-tsubushi at .
White should have recognised that this is a bent four in the corner position and played at himself.