Prone

Overview

In shooters, going prone can make you less noticeable and more accurate, but can also slow you down significantly. Some shooters don't even allow the player to use a weapon while prone.
 
Some adventure games only use the mechanic to pass through small areas.

Prone in strategy games

Some strategy games allows the troops to prone, when the soldiers do this this might imply a reduction of the damage caused by enemy fire, but it slows down the unit as well if it has to crawl, this action can be automatic or triggered by the player.