Coil and strike position!! 'S' shape!!

Before you read..
nothing to be scared of after reading the description, as once you see them in such stance just keep the distance and they'll move away from you..

Attacking stance: All of our pit vipers can bite and effectively inject venom from almost any position. Up, down, or sideways. They need not take time to coil before striking out. They can turn and bite — while they are moving, if they are stretched out immobile or if they are forcibly restrained and have sufficient length left to reach their objective. They can bite in or under water. Although they may not be able to strike very far because of the lack of resistance.

When they realize a potential hazard is near and they feel they cannot escape, most pit vipers will move swiftly into a defensive coil. From this position they can deliver their most effective strike. Normally they do not reach out more than one-third to one-half of their own length. A pit viper's strike is a swift stabbing lunge that is sometimes, but not always, accompanied by an attempted closing of the jaws as the fangs penetrate the target. Although not as fast or accurate as legend would have us believe, the strike is very swift. Too fast to be fully followed and comprehended by the human eye. Stamina and endurance though are not a part of the pit vipers bag of tricks. Repeated harassment of individuals by predators (human or otherwise) will soon wear the snake down to a point where his strike seems almost a slow motion attempt at his former prowess. When passively interested in obtaining a meal, pit vipers will often take a resting coil position near the possible food source. With almost imperceptible movements, this resting coil can be swiftly and easily converted to a striking coil when a prey animal appears.
Source: http://www.rattlesnakes.com/info/strike.html