The Porky Twanger defines the act of inserting a digit up the female corn hole during a session of Bumping uglies. This is optionally accompanied by the calling of the phrase 'Porky twanger!' (spoken aloud at volume), and most likely followed by a swift dismount.
'I got really drunk last night and ended up in bed with my ex-girlfriend. I was doing her from behind, so I thought I'd give her a porky twanger while I was at it.'