A 24th-century Klingon painstik
The painstik (or pain stick) was a Klingon prod-like device used to inflict pain for both ritualistic and submissive purposes. The potential of their use was once described by Miles O'Brien who once saw a painstik used against a two-ton Rectyne monopod. According to O'Brien, the painstik caused the creature to jump five meters at the slightest touch, before ultimately dying from excessive cephalic pressures: its head exploded. (TNG: "The Icarus Factor")
Klingon guards used painstiks on Captain Jonathan Archer when he refused to be silent during his trial on Narendra III in 2152. (ENT: "Judgment")
22nd century painstiks
Painstiks were notably used in the Second Rite of Ascension ceremony, where, in the final part, a young Klingon must walk between two lines of Klingons bearing painstiks which are then utilized as part of a spiritual test of inner strength as endured through physical suffering. (TNG: "The Icarus Factor", "Firstborn")
In 2365, Wesley Crusher, Data and Geordi La Forge created Worf's Second Rite of Ascension ceremony in honor of its tenth anniversary in a holodeck on board the USS Enterprise-D. (TNG: "The Icarus Factor")
Painstiks were also used in the Sonchi ceremony to confirm the death of a Klingon chancellor. The chancellor's body is prodded with painstiks by the Arbiter of Succession and the contenders to become the new chancellor while issuing verbal challenges; the lack of reaction to the intense sensations of the painstiks serve as part of the confirmation. (TNG: "Reunion")
Painstiks during the Rite of Succession
In 2370, Deanna Troi joked about having talked William T. Riker out of planning Worf a surprise birthday party on Worf's behalf, citing "I know Klingons like to be alone on their birthday. You probably want to meditate, or hit yourself with a pain stick or something." (TNG: "Parallels")
The Ritual of Twenty Painstiks was a portion of the ceremony of the Day of Honor. (VOY: "Day of Honor")
After losing to Tom Paris in a game of durotta, B'Elanna Torres stated that she found the game to be far too subtle for her, leading Paris to joke that her idea of subtle is anything that doesn't involve Klingon pain sticks. (VOY: "Night")
During the wedding ceremony of the "Silver Blood" copies of Paris and Torres, the ceremony officiant, Captain Kathryn Janeway, stated that B'Elanna had asked her "to forgo the rigors of Klingon pain sticks in favor of a more traditional ceremony," which allowed the best man, Harry Kim, to chime in with "they're saving the pain sticks for the honeymoon." (VOY: "Course: Oblivion")