is protected by the circled white stone. Whie gets sente. All 4 circled points are black territory.
White throws in with . captures. protects the territory. Later, Black will protect at a and a 1/3 point ko is left. Black makes 2 circled points of territory, plus a captured stone, minus the 1/3 from the ko, for a total of 2+2/3.
White gains 4/3 points but loses sente. Given that this play has a net gain of 4 points (a miai value of 2), there are likely more valuable plays elsewhere on the board. (Comment by Dieter during WME, please correct). To calculate the value:
White gains the circled point with . Later, a is sente to remove another point of Black territory. The difference is 4 points, with a tally of 2.
Black can simplify matters by playing , securing 3 points. Later, he can play a and remove a White point in sente. This is equivalent to the solution, unless this is the last endgame play on the board, but in that case White has good reasons to throw in.