Ah here, I found a better explaination of what my trainer said:
"Since the horse's head is very steady in trot, practically immobile, there's no need for elasticity in the sidereins. The horse is not uncomfortable.
Doughnuts were developed for canter work on the lunge. But I find correct canter on the lunge so difficult for horse and trainer alike that I suggest we leave it for the great masters, along with the doughnuts.
Elastic inserts are not a good idea. If you adjust them so they're not stretched at working length, the contact they provide is insufficient and when the horse tries to find a bit of support they simply stretch. If you adjust them stretched at working length, the horse is trapped. He can't find steady support by stretching to the bit or relief by nodding inwards. Later, under saddle, the horse will not nod "yes" to your hands, because he won't know there's relief inwards. He will nod "no" as a result of the increased pressure, and he'll not be to blame." -HorseAdvice.com, meber Christos Axis