Cu knew Aife was gonna beat the shit out of him. He tried anyway, and she smacked him around like... Well, they're Celts in pre-Roman Britain, so "red-headed stepchild" is probably the wrong idiom. The only thing Cu did right in that fight was to ask Scathach beforehand: "Hey, Teach, what does this lady love more than anything else in the world?" And Sca told him: "Her horses, chariot, and charioteer."
So when she had him disarmed, at her mercy, and beaten so badly that she snapped his sword off at the hilt with one, brutal blow, her guard was down. Knowing that she wasn't expecting him to do anything but die, he looked behind her and shouted, "Oh, shit, your chariot just drove off that fucking cliff!" and when she turned around, he put her in a submission hold, then carried her off to his army's camp.
I'm a bit confused about the "life for life" line, because it sounds like she spared him at some earlier point and is calling in that debt, but it's equally possible that she's just saying that if he spares her life, she'll owe him something of equal value. I don't know either way, because the older a myth is, the weirder people talk in it. That's what happens when you try to translate old, dead languages into modern English, especially when those sorts of things were written that way because they were recording an oral tradition, and oral traditions were kept consistent by using rhyming and alliteration and other mnemonic devices so that they were easier to remember.