Get drunk, do a line, pop a bean...BITE DOWN.
The term "bite down" or "biting down" refers to the clenching of the jaw on hard drugs, hence the saying "bite down." This most commonly refers to ecstasy. Ecstasy tightens your jaw and makes you clench or hold your jaw together tightly. This is a common side effect of cocaine and amphetamines, as well.
When peaking on a drug you do not notice you are biting down sometimes and do not realize what you are doing to yourself until the comedown or the day after. Your jaw can be stiff and in pain after the fact. The drug makes you feel so good you do not notice any pain while you are actually on a drug.
I have experienced the worst of this. After popping a bunch of ecstasy, the next day was rough. I could not eat, not because of the x effects, but because my jaw could barely chew anything without being in excruciating pain. Not only did it happen this time, but after any night of poppin' pills or doing lines.
The best way to avoid biting down is to chew gum. Chewing gum for too long can also be just as bad so take breaks from the gum, as well. Many ecstasy users and addicts will use a baby pacifier to prevent jaw clenching...this is a really bad move to do if you are any where but inside because it is know to police officers that pacifiers are a sign of x using, and they will search you.
Just be aware to relax your jaw and don't play with it if it is stiff...
...once you pop, the pain don't stop.