A shiney Bar costs $10k
A man with alot of bars says... hey!.. i'll let you put a sticker on that bar with your name on it, and in 30 days i'll buy the bar back off you. (and you'll make the difference on what that bar is worth).
Lets kick it off.... but i need you to put down $1k for the sticker... cool?
You have $5k, so you have enough to put 5 stickers on some of his bars.
(In this way you are leveraging your purchasing power).... you just used $5k to put stickers $50k of bars! fk yeah! thats awesome.
Then comes a margin hike... the dude with the bars wants $2k per sticker... you'll need to do it by wednesday.
ahhhh fk... you fker... now i have to sell some of my bars to cover that sticker.... fkr.
get it?
So why have margins? well really... just look at it as a % of the fiat value of that bar. Things would be explosive without these margins. So they are needed to avert self-induced compounding leverage runaway.
big words... but back to the example... if gold goes to $100k a bar... your $5k now puts stickers on $500k of gold.
Thats some scary shit right there. A small % change will wipe you out.
so yea, look at it as a %, and it all makes sense to continually adjust margins.
Where the manipulation can come, is >knowledge< timing of these changes, which is decided buy a bunch of 'dudes' who do coke n hookers with the bar dudes.... (in the example)
(ofcourse)
edit: keep an eye on % calculations:
http://www.bullionbaron.com/2011/03/cme-hikes-silver-margin-requirements.html