Prison my heart in thy steel bosom's ward, But then my friend's heart let my poor heart bail; Whoe'er keeps me, let my heart be his guard; Thou canst not then use rigor in my jail. (9-12)
This is where our speaker offers to sacrifice himself to his cruel mistress so that she'll stop torturing his friend. He says something like, "Go ahead and keep my heart in your heart's jail, but please let me 'guard' my friend's heart." What's odd is that he imagines his own heart as a jail within a jail, which means his friend is his prisoner. In other words, the speaker's fantasy is to keep his friend closer to him than the woman involved in the love triangle.