Drain Knowledge was a telepathic dark side ability used to extract knowledge out of a sentient without his or her consent.
Beyond simple reading of thoughts, the Drain Knowledge ability disrupted the Force to steal deep memories and information in a violent and merciless way. Such a process might damage the subject psychologically.
An individual being subjected to Force questioning
Revan used this power on The One, in order to learn Rakatan during his initial search for the Star Forge, and forced Basic in his brain, which enabled the Rakatan to communicate with him in Basic. Kreia used this power on Atton Rand to learn of his mysterious past in order to blackmail him. Jedi Master Atris may have threatened to use this power on Meetra Surik to force her to reveal what caused the destruction of the Peragus Mining Facility while she was questioning her.
The Sith Emperor used this ability against Revan for 300 years after trapping him in stasis, to learn of the Republic's strengths and weaknesses so he could plan a more successful war. Darth Silthar used this on Tusken Raiders to learn the location of a spiritual map.
Darth Maul, while rendering himself invisible, employed the technique on two Coruscant Security Force investigators to see if they had found a holocron crystal belonging to the late Hath Monchar. While holding Obi-Wan Kenobi prisoner on Geonosis, Darth Tyranus used this power to find any weaknesses in the Jedi Master's defenses. Darth Vader used this ability against Luke Skywalker during their second duel to learn that he had a twin sister. Jerec used it against Qu Rahn, learning that the location of the Valley of the Jedi rested with the late Morgan Katarn.
Kyp Durron (while under the influence of Exar Kun) may have used this ability to rip or otherwise remove scientific knowledge (pertaining to the Sun Crusher) from the mind of Qwi Xux. Jacen Solo used it on former Queen Mother Ta'a Chume to find out if she had dealings with the Dark Nest to kill his child, Allana. His use resulted in permanent brain damage, leaving Ta'a Chume comatose. Darth Talon used it on the defeated Imperial Knight Elke Vetter to gain information on the escaping Princess Marasiah Fel.