Complex sentence: As he moves over dice, they roll in the direction he runs, and creating a group of dice with identical pips, the size of which is at least the number of pips, causes those dice to be cleared from the field.
Split into simpler sentences: As he moves over dice, they roll in the direction he runs. Creating a group of dice with identical pips--the size of which is at least the number of pips--causes those dice to be cleared from the field.