But if people instead regard these excess savings as delayed income, then the cash hoard represents stimulus that has not yet gone to work, to be unleashed when the economy fully reopens.
but、and、or(很少情况下)、for 作为连词可以用在句首,起到承上启下作用,所以 but 和 if 没有关系。to be unleashed 是 stimulus 的非限制性定语,也可以改写为非限制性定语从句:which are to be unleashed...。可以理解为进一步解释说明。