[qaitakalnin]

I dont have this for leathercraft, but i have the same thing in clothcraft called spining.

Basically it means that you get to use 1 crystal to do 3 sytnths when you use the tool, in the case of leather crafting it is callled a tanning vat. It saves space and time and is well worth the investment (has already saved me tons of money in lighting crystals)