More hits on headcrush

Ive heard you can do up to 8 hits on headcrush but I only get 4 and sometimes 5 by mashing. Anyone know the proper way to get a clean 8 everytime? And also if anyone knows how many hits till you cannot DHC anymore it would be a great help. Thanks!!

Basically, if you want that last (& most painful) hit, you can’t DHC.