-elasticity is the hair's ability to stretch out some, yet retain its original length when released from whatever force is acting on it.
-elastic hair has bounce, body, better curl formations, and styles much better. 
-a single strand of hair can hold 100 grams of weight without breaking and the average head of hair can hold 10 tonnes!! 
-healthy hair will stretch around 20% when dry, and up to 50% when wet. 
-be careful with the tension you put on wet hair, (combing) as it is easier to exceed its yield strength than when it is dry.
-if the individual strands on your head have a thicker diameter, your hair is likely more elastic than your fine-haired friends.
-elasticity is dependant on;
        a) hydrogen bonding between both water molecules and keratin strands
        b) disulfide bonds between cystine amino acids. This means that you can improve your hair's elasticity both by adding protein treatments to your regime, and by making sure it has adequate moisture. 
-speaking of moisture, did you know there IS such thing as too much? The swelling induced by water inside the strand puts stress on the outer cuticle. If you are a swimmer or wash your hair every day, consider a pre-wash coconut oil treatment. 
-ultimately elastic hair is healthy hair; and the best way to preserve it is to limit the amount of stress hair undergoes, in every way, shape, and form.