By now, you know that Bertram says he'll never be a husband to Helen unless she can fulfill two conditions:
When thou canst get the ring upon my finger which
never shall come off, and show me a child begotten
of thy body that I am father to, then call me
At this point in the play, Bertram should probably know better than to challenge Helen. After all, the girl has already tracked him down in Paris and has managed to cure the king in order to be with him, right? In other words, Helen's like Frodo Baggins on a mission and she's definitely up for the challenge.
With Diana's help, Helen is able to pull off what's called a bed trick, where one sexual partner is secretly substituted for another. Here's how it goes down: Diana agrees to sleep with Bertram if he'll give her his ring (4.2.9), and then Helen takes Diana's place at the last minute without Bertram knowing. (No, you didn't miss a big steamy sex scene; it happens off stage, somewhere between 4.3 and 4.4.)
Yep, that's pretty messed up. Still, Shakespeare's audiences seemed to have really loved this kind of plot device. In fact, literary critic Marliss C. Desens counts at least 44 plays of the period that use some kind of bed trick (source.) Crazy, right? Apparently, audiences enjoyed seeing unfaithful men like Bertram learn their lessons about trying to cheat on their wives.
Think about what happens in Shakespeare's Measure for Measure: Angelo tries to hook up with a virgin named Isabella but gets tricked into sleeping with Mariana (his jilted fiancée), which seals the terms of their engagement and possibly their marriage. Come to think of it, this kind of stuff isn't so different than what drives modern TV shows like Cheaters or talk shows that are geared toward humiliating cheating spouses on national television.
Now, how did Helen and Diana possibly pull this off? Even if the room was pitch black and Helen and Bertram didn't speak to each other the entire time, are we really supposed to believe this could actually happen? Well, the bed trick may not seem realistic or even possible, but that's not really the point.
According to famous Shakespeare scholar Harold Bloom, the bed trick is Shakespeare's way of making fun of men who sleep around and don't really discriminate between sexual partners. Makes sense to us. Even Helen seems to think so. When she replays in her mind her steamy hookup with Bertram, she wonders how it's possible that Bertram could have made such sweet love to a person that he supposedly hates (Helen). In the end, Helen chalks it up to lust:
But, O strange men!
That can such sweet use make of what they hate,
When saucy trusting of the cozen'd thoughts
Defiles the pitchy night: so lust doth play
With what it loathes for that which is away. (4.4.2)