I'm reiterating some of what's in the Barker's book so keep in mind that's the best source for this info (my memory is only as good as what happend 10 minutes ago )

Basically there are 4 shed cycles for ball pythons.

The first shed is the development shed.....they begin to develop follicles. This happens whenever the female's body triggers it....could be she just wanted it to happen...could be due to the temp drops. Etc.

The next shed I'm not sure exactly what pivital point it is in the cycle but it furthers the growth of the folicles.

The 3rd shed is the pre-ovulation shed. This is where you will start to see alot of building in the female and eventually ovulation. Ovulation is where the folicles decend into the ovum ducts and become fertilized with the retained sperm from all the mating that took place previously....this is where the folicles become eggs.

Finally the 4th shed is the prelay shed where the female is officially carrying eggs and will lay them within about 30 days from the 4th shed.

Slugs are the result of prematurely ovulated folicles. Basically folicles develop asynchronously (independently of one another) so one can be further developed than another. When premature ovulation happens the folicles ovulated too quickly are the ones that become slugs...the others that are farther along move on to become eggs. This is why you get clutches with a mixture of eggs and slugs.

Top reasons for premature ovulation in that 3rd shed cycle:
Handling/palpating before ovulation
Feeding before ovulation
Temps above 82 degrees before ovulation

Again this is what I recall from reading Barker's book and from conversations I've had with Tracy. I'd highly suggest getting it. A great read and TONS of great info from 2 very prestegious and knowledgable people.

Hope that helps!