Pinning is also a term used in both military terms and the expression of them in tabletop gaming such as Warhammer 40k by Games Workshop.

Pinning refers to the reaction of troops whilst they are under fire from ballistic weapons. When fired upon, the natural instinct is to seek cover, but more importantly, if no cover is available, to remain in the same location and position with a lower profile.

Personal theory: The justification for this, in my rationale, is the old chestnut "lightning never strikes twice" - if you survived being shot/under fire, they aren't going to hit you if you stay still

This results in the effective immobilisation of the troops until either the firer runs out of ammunition, misfires, or the troops regain their nerve enough to continue by firing back or moving position (usually to more cover).

The practical (aggressive) applications of this are referred to as covering fire.

Examples of this practice can be found in combat either in live-fire Army exercises or civilian games such as laser tag and Skirmish/paintball - allowing for the team with the better initial firing position to lay down covering fire and protect the forward members of the team, allowing more advanced firing positions to be established. Another example is demonstrated in tabletop war games available from Games Workshop. Initially found in their now-discontinued Hive world game Necromunda but also seeing rules ported to some of their other games. The typical application of this is that if a model (representing a troop) is fired upon AND HIT (but not necessarily wounded), that model is considered "pinned" and may not move or fire back until they are missed or recover their "nerve" - usually represented by a roll of the die and the result being above a certain number. Thus, if pinned, that troop is unlikely to move soon, but eventually will be able to overcome their fear of being shot and move out to either fire back or seek cover.

2003.6.27@13:38 lj says re pinning , an interesting aspect of necromunda's pinning rules was that a character got to take an initiative check to escape pinning at the start of his turn if an unbroken (non juve) friendly model was within 2". This means that it is much more difficult to pin down groups of fighters without using overwatch (covering fire!) to shoot them on their own turn before the movement phase.