Scartol: When Black takes the marked white stone, allows White to take back the three black ones immediately. Now Black has to defend at a.
SAS: This is correct, but obviously Black would play at a, and leave the capture as a possible ko threat.
unkx80: More likely we would be looking into the aji when stones are not removed from the board, such as the possibility of Black playing at a followed by b, making an atari. Usually we do not play ko threats like those in the previous diagram.
SAS: Yes, this is true. I was ignoring the outside because I thought we were talking about the endgame situation. As it is now, Black shouldn't play anyway, since it's too small.