To "defang the snake" means to destroy your adversaries offensive capabilities, limb destruction(especially weapon hand) & weapon disarms.The term is most commonly attributed to having originated in the Filipino Martial Arts such as Arnis and Kali & is usually referring to striking the weapon hand to make the opponent drop the weapon or damage the limb to the extent that they can no longer hold or use the weapon effectively.In various unarmed Martial Arts, this same idea commonly involves using a defensive technique against the opponents attacking limb.It may also be delivering your own strike to the limb of your opponent to try to inflict enough damage to the limb so that it is no longer effective for attacking.
The most popular unarmed defanging technique is maneuvering your elbow in position so that your opponent smashes his fist into it.The ability to accomplish this takes training/practice and a good sense of timing as well as waiting till the last moment so as not to telegraph the movement.It will usually be the lead elbow that you use and starting from a ready/guard position the movement- bring the elbow up pointing to the front/alongside the head just as if you were doing a covering block against a quick hook punch.Put your hand over your ear with your elbow pointing straight ahead and you'll be in the correct basic position.
Depending on the type of technique & angle of the strike,you may bring the arm across the face as if your doing a backfist.The elbow & shoulder is held high while the hand will be higher or lower so as to not obstruct vision.Hand is open and facing outward to provide added protection.
The arm/elbow angle depends on your opponents attack and you adjust accordingly.You MUST also use body as well as head movement with the technique to give you the proper angle as well as the distance to not only perform the technique,but to also provide a margin of safety should your technique be less than ideal.The elbows can also be used against kicks,but usually only the feet/ankles since you probably wouldn't want to jam a full power roundkick at the shin with the point of your elbow.I'm not saying it won't work,but usually not the best idea,especially against a bigger/stronger guy.
You can also strike an opponents hands and forearms(especially if he's covering up)to limit his upper body mobility and striking effectiveness.Low kicks and even grappling techs(especially small joint manipulation) directed to limb destruction as well as gun disarms are all classified as "defangs"in my book even if it's not along what many traditionalist & many Filipino Martial Artist might label them.
Defangs are just another defensive tool that may or may not come in handy depending on the circumstances at hand,but in my experience it has served me well on many occasions and I recommend anyone interested in self-defense to take a look at it.