Vampire Tap

A vampire tap (also called a piercing tap) is a device for physically connecting a station (i.e. a PC, a printer, or another device) to a network that uses 10BASE5 cabling. This device clamps onto the cable, forcing a spike through a hole drilled through the outer shielding to contact the inner conductor while other spikes bite into the outer conductor. From the vampire tap, a short cable called an AUI (Attachment Unit Interface) is connected directly from the tap to the network card in the PC. Vampire taps allow new connections to be made on a given physical cable while the cable is in use. This allows administrators to expand bus-topology network sections without interrupting communications. Without a vampire tap, the cable has to be cut and connectors have to be attached to both ends.

Source: http://www.blackbox.com/resource/files/applicationdiagrams/athickethvampconnc.GIF