Mindbending was a form of psychic wrestling played by Time Lords, utilising an imaging device as connection. Usually it was only a game, but it could end in deathlock. The competitors placed their heads against the mindbending device and the process began. It involved mental warfare, hence, "mindbending".
The Fourth Doctor and Morbius entered a mindbending contest on the planet Karn with the former eager to prove the latter's mental instability following his revival. Faces appeared on the holographic screen between them, showing the Doctor's first three incarnations, eight of the Doctor's earlier lives, and one face that belonged to Morbius. Both were badly hurt by the fight and Morbius was forced to retreat before being forced over a cliff by members of the Sisterhood of Karn. The contest left the Doctor dying, but he was restored when the Sisterhood had him drink the Elixir of Life. (TV: The Brain of Morbius)
After being trapped in the Matrix by the Spy Master, the Thirteenth Doctor escaped by overloading the system with memories of her past incarnations and of all she had seen; the eight faces, as they had appeared to the Fourth Doctor during the mindbending contest, were among them. (TV: The Timeless Children)
Behind the scenes
During the mindbending sequence, several faces are shown. They were images of the production team. The appearances of these images has led to several theories concerning the Doctor's past regenerations. These pre-Hartnell incarnations of the Doctor have come to be known "Morbius" Doctors. They would appear again, through archival footage, in 2020's The Timeless Children.