It is a very good thing, between exercices and at the end of work, to stretch the topline of your horse and release it from any tensions.
But you still have to do it in a thoughtfull manner, keeping soft contact, a relaxed jaw and a poll free of tensions. Then, you have to make sure your horse is not moving his balance to the forehand.
Stay focus on your circle, or on your line and make sure your horse is also straight and does not collapse on any side.

While keeping this chewy contact, try to have the horse open his frame, more than just having him go down.
Not only will this exercise improve throughness, it will also improve the complicity between your hands and the horse mouth - having a horse that trustfully follow your hand, wherever it is.



Here are two pictures :

INCORRECT STRETCH - horse weight too much on the forehand, low activity from the hind legs, neck could use more lengthening.

BETTER STRETCH - horse more balanced, good activity of the hindlegs, topline gently working. Neck could lengten a bit more, but that's still good considering the early stage of training - hence the petting !