The Compound Effect?

    ... and presently Jack Aubrey, in a summer-house by the bowling-green, began again, playing very gently in the darkness, improvising wholly for himself, dreaming away on his violin with a mastery that Stephen had never heard equalled, though they had played together for years and years.
    Like many other sailors Jack Aubrey had long dreamed of lying in his warm bed all night long; yet although he could now do so with a clear conscience he often rose at unChristian hours, particularly if he were moved by strong emotion, and crept from his bedroom in a watch-coat, to walk about the house or into the stables or to pace the bowling-green. Sometimes he took his fiddle with him. He was in fact a better player than Stephen, and now that he was using his precious Guarnieri rather than a robust sea-going fiddle the difference was still more evident: but the Guarnieri did not account for the whole of it, nor anything like. Jack certainly concealed his excellence when they were playing together, keeping to Stephen's mediocre level: this had become perfectly clear when Stephen's hands were at last recovered from the thumbscrews and other implements applied by French counterintelligence officers in Minorca; but on reflexion Stephen thought it had been the case much earlier, since quite apart from his delicacy at that period, Jack hated showing away.
    Now, in the warm night, there was no one to be comforted, kept in countenance, no one who could scorn him for virtuosity, and he could let himself go entirely; and as the grave and subtle music wound on and on, Stephen once more contemplated on the apparent contradiction between the big, cheerful, florid sea-officer whom most people liked on sight but who would never have been described as subtle or capable of subtlety by any one of them (except perhaps his surviving opponents in battle) and the intricate, reflective music he was now creating. So utterly unlike his limited vocabulary in words, at times verging upon the inarticulate.
    'My hands have now regained the moderate ability they possessed before I was captured,' observed Maturin, 'but his have gone on to a point I never thought he could reach: his hands and his mind. I am amazed. In his own way he is the secret man of the world; but I wish his music were happier.'

From The Commodore by Patrick O'Brian (Book 17 of the Aubrey-Maturin series)

Is this perhaps what Darren Hardy and Jeff Olsen described in their books?