What is the connection between Charity and Virtues according to Aquinas?

According Paul Wadell's book *The Primacy of Love: An Introduction to the Ethics of Thomas Aquinas*, Aquinas defends that Charity is the mother of all virtues, and also how they can be perfected into 'gifts' of the Holy Spirit. It is exactly this part of how charity perfects them into gifts that I do not understand. It says that charity, when faced with its human limit, with its limit as a virtue, it is transformed by God into the Spirit, taking the virtue to its plenitude. How does this transformation happen? Or is it a mystery?