Whip landing on the slave's back makes him service Mistress's feet better
The shape of Mistress Tiffany's feet is totally blameless - good enough to make her slave's mouth water. These feet are his personal source of pleasure as well as his personal source of pain. Those spike heels sinking in the soft flesh of his crotch, his stomach, his face... They make him writhe and bellow. The pain is followed by pleasure again though - see him licking Mistress's bare feet here.