Hi Anne, this is an interesting query. It would be hard to clarify exactly what was meant without hearing the performance of course, but I would think that you're right and that the use of this phrase in the context you mention refers to a reservation regarding control of tone/sound. From how your pupil normally plays you may already have an idea what this could be, but nerves on the day could have affected your student in a way you've not heard before, so do ask them - and maybe the accompanist too - what they think? in the context of dynamic contouring, a swell could well be positive, as you say; on the other hand, if the change of sound is abrupt, misplaced or results in a loss of consistency of tonal colour, I can well imagine how it might have a less desirable effect on the musical outcome. It's really heartening to hear how you're considering how you can help your student address this feedback in the future, and it sounds like your pupil did very well already - congratulations!