By the end of the play Lena has come to the realization that she's a lot stronger than she always thought. This puts her in a position of relative strength with regards to Boesman, who'd previously subjected her to so much verbal and physical abuse.
With Outa lying dead, and with Boesman needing to make himself scarce as soon as possible, Lena pointedly refuses to help her traveling companion pack up and leave. Instead, she taunts Boesman, realizing that he's been weakened by his killing of Outa. Now that Boesman is so obviously vulnerable, Lena senses a radical change in the dynamic of their relationship. She senses that it is she who now holds the power.
To a large extent, this is because Boesman, after kicking Outa's dead body, now knows what it looks like to be beaten, as he'd beaten Lena on so many occasions. Confronted by the evidence of his gruesome handiwork Boesman immediately starts to panic, grabbing all his belongings with the intention of leaving the hut immediately and taking to the road again. Though Lena will go with him once more, this time she won't lie down to Boesman; she'll stand up to him. As she says to him right at the end, the next time he hits her it should be hard enough to kill.