seems to be an effect that works from muscle exposure, affecting the metabolism of local fat cells, but the effect is completely and completely irrelevant in quantitative terms. The amount of fat mobilized due to increased hormones or blood flow is just negligible and doesn't matter in the real world.

There is also the fact that, compared to something like cardio, localized exercises burn a small amount of calories. Doing cardio for 30 minutes while burning only 5 kcal/minute (very easy) burns 150 calories in total. Given that 90% of fat is in energy expended, then you burned 15 grams of fat. This is compared to the 0.1 gram of fat that can be mobilized by doing crunches or leg raises.

In addition, all body activities will have an impact on fuel and hormone use in a much broader way affecting lipolysis and blood flow. Spending an hour on localized exercise pales in comparison to the fat-burning effects of even moderate cardio. Wasting time on abs or glutes/thighs is just wasted time in terms of any localized fat burning.