The theory which makes sense to me is that the boiling process, also occurs in each individual wood cell. This causes a rupture of the cell wall of every cell in the chunk of wood. Following boiling, slow drying by putting the chunks in a standard paper grocery bag and putting them aside for a few weeks results in pen blanks, carving blanks with little or no cracking. The moisture in the center of each chunk can more easily escape to the surface due to the cell wall rupturing. The "even" ambient humidity provided by the paper bag helps assure the surface does not dry more rapidly than the center. With differential contraction/shrinking virtually eliminated there is no (or greatly reduced) cracking. The rule of thumb is high rolling boil for 1 hour per inch of thickness of the biggest chunk in the "boil". Technique works well - indispensable for apple wood!