The idea is that the mugger is in a rush. They don't know if you really have nothing or whether you're lying. They might rough you up to get you to admit you ha

The idea is that the mugger is in a rush. They don't know if you really have nothing or whether you're lying. They might rough you up to get you to admit you have money. And once that happens, if you react defensively, they might get more violent. The other possibility is that they might ask for your whole wallet/phone, etc. Whereas if you hand over some money they'll often take it and go. At least this is what Brazilians who had been mugged told me happens. Some carried a dummy phone to hand over as well.