Excerpted from Speaking of Jazz: Essays and Attitudes:
Contrafacts are good for their expediency, since you can come out with your own line on a standard tune without having to first rehearse it. The idea worked for Bird, and it is still done today. Since these progressions are familiar to the rhythm section, contrafacts work with ease on the gig—as long as you know your Head line. Yet, they are fresh to the audience. Seen below is Bird Call, my contrafact on Charlie Parker’s Confirmation. The exposition line is based on the chromatically stepwise descending guide tone line, mixed with the second guide tone line. There is a send-off for the drum solo based on the same line.