Difficult to understand what it means. In letter it was said after winter when spring is starting. This is difficult time to do anything. Hive is quite small and mites in brood cells.
If you have too much mites, take brood away. It is difficult decision.
Second point comes when yield is over and bees raise winterbees. Mites consentrates into winterbee cells and it is time to give treatment.
I do not count mites. I handle hives every year and do not let them make me surprise. It have happened however every now and then.