Headcanon: Andalites can project a burst of mental noise, emotional noise, whatever you want to call it or however you want to flavor it, directly into their opponent's head to gain an advantage in a fight. It could range from a brief jumble of mental noise that throws your enemy off, to a whole psychic attack meant to destabilize or even paralyze your enemy. It makes sense as a defense mechanism, just like how humans can scream to intimidate predators. Just beam a bunch of noise, or fear, or the memory of pain, or something along those lines, straight into your opponent's head to briefly fuck with them, which might give you an opening to make a strike or to try and run away.
I don't know if this is a "Young Andalites who are Frightened" trick, a "Any Andalite could do it if they think of it" trick, or a "Psionic masters like Book 1 Elfangor can do it" trick. We know Elfangor could project memories, visual information, and even emotions into the heads of the Animorphs, so at the very least he could probably do this. However, the fact we don't see it in fights may mean it just takes too much concentration to bust out when all your attention is on tail fighting.
