Matā is a conditional noun meaning “whenever.” Tarjum is the conditional action and tazdad its result. The pronoun bihi refers back to the nose or forward part of the head mentioned in the description.
The sentence conveys more than the general fact that the animal moves fast. It establishes a particular sequence: when she thrusts her head down and forward, her pace increases. That conditional link moves the poem from physical portraiture into the kinetic description of travel.