Your teacher maybe refering to the insight that can occur during meditation but I'd like to widen this to include any Buddhist meditation methods or variations thereof. Also, the unearthing of deep-seated unconscious material can produce unpleasant mind and behaviour altering states.
The Buddha's teachings can be quite potent for some and respect must be given to the process of unfolding insight.
There is a sutta that describes the power contained in the teachings and it's probably the most extreme example. (Vesali Sutta, SN 54.9.) The Buddha had given the monks a practice before going away for two weeks. When he came back some of the monks had taken a knife to themselves and ended their lives. They had fallen into the extremes of the disgust of their own bodies.
It's wise therefore to have a healthy mind and body so that one can hold the fascinating experiences and insights that may arise.