he said she is not wearing any underwear right now.. but actually she is wearing one.. then he statedhis watch is running ahead by 10 minutes ... by this he meant that she won't be wearing underwear after 10 minutes.. might be having sex with him...
he is saying that the girl isnt wearing any underwear but she is at the moment so he says his watch is 10 minutes, so in about 10 minutes the girl wont be wearing any underwear (not sure if i explained that very well)