This is directly from the House Budget Committee:

A 2018 Government Accountability Office legal opinion holds that if the President proposes a rescission, he or she must make the affected funds available to be prudently obligated before the funds expire, even if the 45-day clock is still running. This means, for example, that the President cannot strategically time a rescission request for late in the fiscal year and withhold the funding until it expires, thus achieving a rescission without Congressional approval.