The drive generates a reference voltage proportional to speed. This voltage is sensed by an offset/gain circuit that becomes linear (from 0V) at 4 revs a sec and goes non-linear again (clamps at 3.3V) at 6 revs a second. It's output analog ORs with the sine-cosine reference. The sine-cosine reference is always there, it is simply swamped by the ORed full-step morph signal. So big deal, decel to zero instantly. 2 milliseconds later the reference reverts from full-step to sin-cos. Enjoy the 141% torque bump while it lasts for the 2mS. It's all analog.