So your decision doesn’t just determine the future; it also determines (with high probability) which you “you” are.
Worse. It doesn’t change who you are, you are the person being blackmailed. This you know. What you don’t know is whether you exist (or ever existed). Whether you ever existed is determined by your decision.
(The distinction from the quoted sentence may matter if you put less value on the worlds of people slightly different from yourself, so you may prefer to ensure your own existence, even in a blackmailed situation, over the existence of the alternative you who is not blackmailed but who is different and so less valuable. This of course involves unstable values, but motivates degree of existence phrasing of the effect of decisions, over the change in the content of the world phrasing, since the latter doesn’t let us weigh whole alternative worlds differently.)
Worse. It doesn’t change who you are, you are the person being blackmailed. This you know. What you don’t know is whether you exist (or ever existed). Whether you ever existed is determined by your decision.
(The distinction from the quoted sentence may matter if you put less value on the worlds of people slightly different from yourself, so you may prefer to ensure your own existence, even in a blackmailed situation, over the existence of the alternative you who is not blackmailed but who is different and so less valuable. This of course involves unstable values, but motivates degree of existence phrasing of the effect of decisions, over the change in the content of the world phrasing, since the latter doesn’t let us weigh whole alternative worlds differently.)