CaptainMongo speaks the truth.
Only heard it once, and on that occasion, the speed was still well clear of Vls. With a flat pitch attitude turning into the wind on base, our FPA steepened rapidly without a corresponding speed increase as we hit a leeside rotor. I.e. we lost energy, rather than traded it...
The alert sounded before a pitch adjustment was made. ATHR was waking up, responded sufficiently. It is called the "low energy" alert for a reason.
Its all in the FCOM DSC section, and worth a brief anywhere one might mention "terrain induced windshear" or similar.