When printing very high prints, sometimes the "not-so-often-used" parts of z axis rods get dusty and cause skipping.
This is designed to increase the belt-pulley contact and thus increase the friction -> less skipping.

Designed to use the 623 bearings (the same as mendel inspired lowrider by twotimes.)

Even when this is in place I recommend to oil the rods regularly with some gun/sewing machine oil. This too helps reduce skipping.

Print all the parts (you'll need 2 of washer.stl's)
Loosen the belt
Unscrew the bolts from stepper
Insert the m3x20 screws
Wiggle the bearings in placewith the belt in between.
Insert printed washers
Tighten the stepper bolts (make sure the hole is centered)
Put the top on and tighten the nuts
Print very high prints with reduced skipping..