You missed the important part of the timing: "while an Imperial figure is defending."
An ability can only be used once per timing instance. This is described for "while attacking" abilities, but clearly covers while defending too.
Thus, the 'then' is immediate. As soon as you use the ability, pay the threat to ready it or not, but you can't activate it again until you are defending a different attack.
If the attacker has surge=pierce 1 And surge=Cleave and only one surge, can I wait the surge decision before using Power to Shields?