Assimilation tubules, also known as injection tubules, were a pair of flexible needle-like devices that extended from the wrists (female drones and the Borg Queen) or fingers (male drones) of Borg drones. They were the first step in the assimilation process.
Once they penetrated the skin, they released a series of Borg nanoprobes, the bloodstream being the initial target. They took over the blood cell functions in a manner similar to a virus. Assimilation was almost instantaneous. (VOY: "Scorpion")
The tubules were capable of penetrating any alloy or energy field known to the Federation. They also allowed drones to interface with technology and possibly reconfigure it using their nanotechnology. (ENT: "Regeneration"; Star Trek: First Contact; VOY: "Scorpion")