Tension wrench (as used in lockpicking):

The tension wrench (sometimes called a "torsion wrench" or "torque wrench"), occurs as one of the two basic tools of lockpicking. Contrary to a literal interpretation of the name, such a device does not "stretch" anything(1). Quite the opposite, it exists as a tool allowing the user to exert a small amount of torque about the longitudinal axis of an object running perpendicular to the wrench itself. For this reason, the name most likely comes from a corruption of the rarely-used "torsion wrench", which more accurately describes the actual purpose of such a device.

A tension wrench generally looks like this:
                            __
                           / /
                          / /
 ________________________/ /
|_________________________/

Basically, a thin (and stiffer works better) strip of metal, approximately 5 inches long by an eighth of an inch wide, with a 70-85 degree bend near one end. The head (the short portion past the bend) should measure roughly half of an inch long.

The tension wrench exists for one reason and one reason only. It allows the user to put a slight amount of torque on the plug (aka core, aka cylinder, aka the thing that actually turns when you put a legit key in). This causes one or more driver pins (the ones above the plug, in the hull) to bind (catch between the plug and the hull) which in turn allows the user to attempt to pick the lock by raising each pin to the sheer line one at a time.

To quote the MIT Guide to Lockpicking (the Bible on this subject), which refers to the tension wrench by the more apropos name of "torque wrench",
People underestimate the analytic [sic] involved in lock picking. They think that the picking tool opens the lock. To them the torque wrench is a passive tool that just puts the lock under the desired stress. Let me propose another way to view the situation. The pick is just running over the pins to get information about the lock. Based on an analysis of that information the torque is adjusted to make the pins set at the sheer line. It's the torque wrench that opens the lock.

1 - For certain precision fastening needs, something worthy of the label "tension" wrench does exist. These always require hydraulics due to the nature of the task of literally putting a bolt under tension rather than merely securing it, under some arbitrary degree of tension, via friction. Such devices generally carry much fancier names than a mere "wrench".

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