Metabolic adaptation describes how your energy expenditure adjusts to a diet: as you lose weight, the amount you burn each day goes down too.
Where it comes from
Two effects add up. First, a lighter body needs less energy. Second, many people unconsciously move less in daily life while dieting. This everyday movement outside of training is called NEAT, and it often drops without you noticing. Trexler, Smith-Ryan and Norton (2014) describe these adaptations in a review on athletes.
For you this means your deficit shrinks over the weeks, even if you keep eating the same amount. That is a common reason for a plateau.
What diet breaks have to do with it
In the MATADOR study (Byrne et al. 2018) with men with obesity, resting metabolic rate, adjusted for body composition, dropped less with two-week breaks. In the ICECAP trial (Peos et al. 2021) with resistance-trained adults, there was no difference in resting metabolic rate. Based on current evidence, a diet break is therefore more a tool against hunger than a reliable fix for adaptation. Controlled studies showing a metabolic effect of reverse dieting are also lacking.
What you can do
Keep an eye on your everyday movement, for example through your step count. Expect to adjust several times during a long cut, even if you do everything right. The most reliable way to spot adaptation is to compare your intake with the slope of your trend weight, rather than relying on a formula.