power correction, secondary turns, etc


I, too, have been reading Richard Quick's massive amounts of info.  I 
have a couple of questions that some one else could help me with.

Could someone explain how power correction capacitors work?  I only know of 
their "need".  I don't know how they fit in the circuit, what size they 
should be, or why or how they work.  Could someone please clue me in?  I 
(and others) would be grateful.  

Another question:  I now have heard two ranges for the number of turns on 
a secondary.  I have heard between 400 and 600 and now between 800 and 1100.
My coil has 700 turns.  I think this coil works well, so does it even matter
how many turns as long as it's between 400 and 1100?

Coil forms:  I read that PVC is less than ideal for a core form.  How about
HDPE?  I know a source of HDPE pipe of various diameters: Fireworks mortars.
They come in 3, 4, 5, 6, 8, 10, and 12 inch i.d. sizes.  Is there a problem
with HDPE that I don't know about?