The spine, called a caudal peduncle spine, is a modified scale anchored in a shallow groove either side of the tail. Muscular flexion of the peduncle erects it, and the fish delivers the blow with a lateral sweep of the tail rather than a bite. The edge is sharp enough to cut human skin, and handlers of aquarium stock are routinely warned about it.
Unlike some Indo-Pacific relatives, the regal blue tang has no venom gland associated with the spine. Wounds are mechanical lacerations, and the risk is infection rather than envenomation. The bright yellow that surrounds the spine in several surgeonfish species is generally read as a warning signal.
The common claim that all tangs are venomous appears to come from conflating the group with rabbitfish and with the one or two Acanthurus species reported to have toxic mucus around the spine. For Paracanthurus hepatus specifically, the evidence points to a purely mechanical weapon.