This is a good exercise.we also use it to allow the understanding of the stroke
I would do it only one stroke for one length pool head in watter
exercise is good but you have to control the immersion of the head and breathing.we use this exercise when students already have a good floating.We use smaller bearings
I think this is a very good basic exercise for freestyle for swimmers who already know how to breath to the side. For swimmers who swim this exercise with head above water I would first suggest an exercise for learning to breath on the side first.