marrowwalks have lvl 33 bone prison charges on them. as long as you don't put any points into bone prison and you have no other items that give points specifically to bone prison, you'll get credit for lvl 33 bone prison as a synergie in other skills. this is huge because a)it saves you 20 skill points and b) normally the highest synergy you can get is 20 (synergies only count hard points put into a skill, not bonus points)
i use it on my summon/bone hybrid and it gives a huge boost to bone spirit and bone armor. this works with any item with charges, but it's called the marrowwalk bug because 33 bone prison is the highest lvl and most usefull of any of the charged items.