Big Boss using the Crawl ability in Metal Gear Solid V: Ground Zeroes.
Crawling is a gameplay mechanic featured in many Metal Gear games, used typically to sneak past enemies undetected.
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When the player character (Solid Snake, Big Boss, or Raiden) crouches, they will then be able to crawl around. This ability enables them to avoid enemy detection, but also to enter small passageways.
The player can crawl around anytime they want, even when they aren't sneaking around/avoiding detection. They can also crawl backwards.
Old Snake crawling around in Metal Gear Solid 4: Guns of the Patriots.
Due to the shift in genre, from the traditional Stealth to Hack 'n' Slash, the Crawling mechanic is not present in Metal Gear Rising: Revengeance.