AP Style - clarifies 'nerve-wracking'

It's always been a bit unclear from the AP stylebook whether the phrase should be "nerve-racking" or "nerve-wracking."

The "-racking" form is favored by Bryan Garner in Garner's Modern American Usage, for instance. And AP seemed to indicate it favored that with its general wording:

The noun rack applies to various types of framework; the verb rack means to arrange on a rack, to torture, trouble or torment: He was placed on the rack. She racked her brain.

Paul Brians also used this logic in his entry favoring "rack."

If you are racked with pain or you feel nerve-racked, you are feeling as if you were being stretched on that Medieval instrument of torture, the rack. ...

 However,  the wire service has now put out a clarifying note:

The noun wrack means ruin or destruction, and generally is confined to the phrase wrack and ruin and wracked with doubt (or pain). Also, nerve-wracking.

So you are now faced with what seems to be an increasingly prevalent conundrum - go with AP or follow the other sources. AP gives no reason for why it seems to favor the "wracking" construction.