I'm not sure which part your're asking.
1. Why did he wrap the peeled branch?
I'm not sure. Maybe this keeps the cambium from bridging the gap somehow.
2. Why didn't he apply rooting powder?
It's not clear if he scraped the cambium off. If he did, there's no point in applying rooting power on the peeled area, since there's nothing there to grow roots from. If you want to apply rooting hormone, apply it on the cut at the air-layer end. It might grow fast enough that hormone is not necessary. I've never done citrus before but longan grows roots fairly quickly.
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Thanks fyliu. I did the air layered on longan in OCtober, but have not seen any roots. I should open the wrapper to check inside. Maybe in two weeks.