ZigZag Formula

[waheeb <WAHEEB@xxxxxxxxxxxxxx>]

I am trying to write an indicator and ZigZag is one of the components, Does

any one know the formula for ZigZag?
I couldn't understand the turning points of this indicator. 
If I set the reversal amount to be $10, where should it reverse and do you
calculate
it from the previous point, one day before or one day after.

Thanks in advance

waheeb