Its a great trick and I have used it before on the AA oneWorld awards. Good to use also if you need to miss an open-jaw.
Say you've already open-jawed on your trip originating in BOS, but you want to end up in SFO. If you were to officially end your trip in SFO, you'd be breaking rules, but if you fly through SFO-BOS then just get off in SFO you can accomplish it. Of course, in this scenario you'd have to be under your mileage threshold.