Piper decided to jump of the roof. It wasn't a rash decision on her part.
This was her plan--climb to the top of the roof and pick up speed by running from one end all the way to the other.
Jump off. Finally, and most importantly, don't fall.
She didn't make plans in the event that she did fall, because if you jump off the roof of your house and land on your head, you really don't need any plans from that point on. Even Piper knew that.
So that's what she did. She jumped clean off her roof.
But before we get to what happens next, you'll probably need to know a thing or two about a thing or two. . . .