We bless Hashem who kept his promise. This going down to Egypt was only for a set time, established at the Bris Bein Habesarim, that they would go down to a land not their own, and be oppressed for 400 years. And Hashem calculated this time, and waited for it.
And Hashem would of course punish that other nation for oppressing His people, but the end result is that they would leave, and with a great treasure. And this same promise which stood for them, stands for us as well. Because this oppression and yet Hashem's maintenance of a relationship, and a salvation, has stood for us throughout the generations. (Perhaps this is an expression of קרבנו לפני המקום?)
Alternatively, if the shevach was קרבנו לפני המקום of Avraham Avinu, then the bris bein habesarim with its long-standing havtacha is the shevach.
This also relates to the distinction we make, on Pesach, between chametz and matzah. In every generation they arise upon us le-challo-seinu. But Hashem is mazti-leinu miyadam. :)
This transitions into Tzei Ulemad, as an example of how Hashem saved us in every generation.