Xan is fucking around with magic again.
He'd probably object to the phrasing "fucking around," when he's really trying to do something very important. Then again, he might not object at all. Depends on how he's feeling that second.
At any rate, he's fucking around, and then he fucks up. And he's hurtling through a tunnel, and at the bottom of that tunnel is a floor, and it isn't long before he hits that floor, hard, and breaks his arm.