Well, my guess would be she's talking about when she's asleep and her husband starts poking his.....um....ham..at her from behind...sort of between her...um....bread. Imagine she's laying on her side with her back to hubby and he's, well, you know...from behind? Not like IN the behind, just FROM behind....
That's just my guess, I have no experience in receiving a "ham sandwich", for the record. 😉