Here's the snippet from the (now free) Hackmaster Basic:
Throughout these rules you’ll notice that
many references to die rolls are annotated
“dXp”. The ‘p’ suffix indicates that this die
‘penetrates’. Should you roll the maximum value
on this particular die, you may re-roll and add the result of
the extra die, less one point, to the total (penetration can
actually result in simply the maximum die value if a 1 is subsequently
rolled, since any fool knows that 1-1=0). This
process continues indefinitely as long as the die in question
continues to come up maximum (but there’s always only a
–1 subtracted from the extra die, even if it’s, say, the third
die of penetration).
Note that a d20p uses a d6p to compute additional penetration,
while a d100p utilizes a d20p.
