Given nine labeled elements (say ABCDEFGHI), the Nth digit of a number
appears to mean "move the element at position N+D (wrapping around as
needed) to position N". A valid siteswap is one that leaves each
element in a different position. (If the rule was "N-D", then 423531441
would be invalid.)
Less obvious is why these particular siteswaps form a sequence. If we
start with ABCDEFGHI and apply each one in turn, then we get
423531441 -> EDFIHGBCA
692973126 -> BDHIFACEG
825799941 -> GIEDFACHB