We study the barrierless and highly exothermic F + NH3 and F + ND3 abstraction reactions using quasiclassical trajectory calculations based on an analytical potential energy surface developed in our research group. The calculations correctly reproduce the experimental evidence that the vibrational fraction deposited into the DF product for the F + ND3 reaction is greater than into HF for the F + NH3 reaction and that the vibrational distribution is inverted in the HF(v') and the DF (V') products. Of special interest is that recent crossed-beam experiments reported by Yang and co-workers at 4.5 kcal mol(-1) are reproduced for both reactions, with a mainly forward symmetry associated with direct trajectories, and a small sideways-backward symmetry contribution associated with "nearly trapped" trajectories due to a "yo-yo" mechanism, different from the previously suggested mechanism of a long-lived complex.