As you can see from the prior posts, there are a number of methods to try to prevent cracking. Here's what I do for my twice turned hollow forms. When I harvest green wood, I always seal the ends (Anchorseal) right after it's cut. When it's time to turn, if there is any checking on the ends, I cut them off first; start with a defect free blank. In contrast to others, I usually turn with the pith in the center ( I'll get back to my reasoning at the end). When I turn the rough form, the bottom is turned with a 1.5" diameter (approximately) "drive shaft". The drive shaft runs between the tenon in the chuck, and the bottom of the HF. This eliminates the mass of wood at the base that would otherwise dry slower than the rest of the piece. If it's a tall HF, I use a steady rest for the hollowing process. Once the outside shape is completed, I coat it with Anchorseal, including the drive shaft and exposed area of the mounting tenon. I hollow leaving a 1/2" wall thickness, including the bottom. Once the piece is off the lathe, I coat the tenon area that was in the chuck with Anchorseal. I weight it and record the date and weight on a piece of paper that I leave in the top opening of the HF. I then put it in a paper bag and hang it upside down. I'll leave the paper bag on for about a week, and then let it hang exposed to air, and weight it once a month. Usually takes 3 - 4 months for the weight to stabilize.
And now for the issue of the pith. My theory is that the rings around the pith are somewhat concentric and they should shrink down in a somewhat uniform fashion. A blank that is taken out of the log to the side of the pith now has rings that are somewhat "C" shaped. When the wood shrinks, it will tend to pull in opposite directions from the open side of the "C", possibly resulting in a crack. As I say, this my theory. As long as I slather all external surfaces with anchorseal, have a uniform wall thickness, and dry slowly, I have been very successful in having crack free rough turned HFs.