Oshi-tsubushi  

is oshi-tsubushi, literally "squashing push", a Japanese term. See below [1] for translations.

After , White's connection at the marked point is an illegal suicide move under most rulesets. Later Black can capture the two White stones, gaining two eyes and life.

Black instead tries to play ko and dies  

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.)

Behavior under different rule sets

Self-capture as ko threat  

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.

White to play and kill  

If White to play in the original position, he can kill Black by connecting at and forming nakade. If captures the three stones, White 3 at holds Black to one eye.

Damezumari prevents oshi-tsubushi  

Oshi-tsubushi can be prevented by damezumari. In this position, Black's lack of liberties means that after he is still in atari.

Typical problem  

here doesn't understand the position. Now Black can live.

Black 4 lives  

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.

English terms

^[1] There is no consensus on an alternative English term for Oshi-tsubushi. Robert Jasiek in his books uses internal liberty shortage for this kind of life and death tesuji. The everyday translations, crush, squash, and smash, are also occasionally seen. A related concept is Eye by exhaustion, a term coined by Tamsin.