Meaning of the term
is oshi-tsubushi, literally "squashing push", a Japanese term. See below  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.
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
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 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.
Oshi-tsubushi can be prevented by damezumari. In this position, Black's lack of liberties means that after he is still in atari.
Application in life & death situations
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.
 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.