In 1953 the Kenyan Mau Mau offensive started. A long dead friend, an RAF pilot then still flying Spitfires amongst other things, volunteered for secret ops flying unnamed British servicemen into remote parts of the country. One told him of the simple method of getting black Kenyans to confess - a length of flexible piano wire in a running noose stapled to a 6" piece of wood was used to lassoo then garrote the suspect's pulled testicles.
And the same didn't happen at Guantanamo & all the other foreign sites use by America to disappear prisoners of war & others not simply murdered?
Not common. The main technique was much more subtle. The mass of the Kikuyu had a very strong belief in witchcraft, and so were made to take part in a grisly oath of allegiance ceremony, including eating raw testicles and drinking blood, etc. (Note. Early explorers complained about being spat upon by the Kikuyu, not realising that this was the highest honour of trust a man could bestow. He was giving you a part of his body, which you could have taken to a witch doctor and paid to have a deadly spell cast upon him!)
Since they were so superstitious, all one had to do was hold a chameleon near them, as they believed that if a chameleon stared at them they would become sterile. No need to castrate! The oath of allegiance was on the latter years cleansed by "good" witchdoctors in another almost as grisly ceremony. This ploy allowed unwilling Mau Mau to quit and was extremely successful.
Whilst atrocities did occur, many tales were exaggerated.
By the way, I served in the Kenya Regiment towards the end of the Mau Mau period, but had friends who had served from much earlier.