Here is a live recording of him playing the Bartok Sonata, certainly one of the best performances of the piece in its history. In the first movement the part where there are so many voices in different registers of the keyboard going at one time is jaw dropping in its relentless drive. The second movement with its climaxes of blazing, overpowering light amid the glowing quiet passages and the powerful optimism of the last rondo movement ..... it would be better to hear it than to read about it.
But Kocsis was more than "just" a Bartok interpreter, he played a wide range of music with both great originality and great integrity. I'll post more of those performances in the coming days.