By wiping a clean finger on the spare gound and then sniffing it or placing it under his partner's nose, a man thus enables himself to check the current hygeine status of his genital area.
Brenda - "(Sniffs) What the fuck Timothy! Your finger smells like shite! What the fuck have you been doing?"
Timothy - "Nowt. I've just ran it up my spare ground to see if I could get away without going for a shower before I get Knighted at Buckingham Palace today".