What is a jack rafter or called a cripple?

A Jack rafter is a rafter that has the pitch of the roof cut onto one end with a 54 degree angle this sort of rafter is one of several that fit onto a hip ridge.

To cripple a piece of timber means to cut it partway through in order to straighten it if this is done to a wall timber it will need to be splinted to restore the strength of the stud