What is stepper motor binding? (When belts are too tight) Recently I have been getting some layer shifting starting at layer one. I have had layer shifting at higher layers due to various reasons but mainly for the belts being too loose. But now I am reading that layer shifting can also be caused by belts being too tight.

The RepRap wiki page for layer shifting simply gives the mechanical reason for this as "binding". Can anyone explain what binding means? I thought it meant that the rails were crashing into something but apparently it doesn't. Then I thought it meant that the X and Y axes weren't perfectly perpendicular. 

Does it mean that the "teeth" of the belts stay "stuck" to the gear for too long when moving in one direction? Why would this happen in one direction and not the other? Because the pulleys/gears are at different heights? Or just because of the belts being tighter? Or one of these reasons?

Just trying to understand what its happening so I can debug it for my particular 3D printer.
I think the RepRap wiki is using the word "binding", which translates to "stick together or cause to stick together in a single mass" (from Google dictionary), to indicate that some sort of friction is experienced (as you experience when things are sticking together).

When there is too much tension in the belt, pulleys and bearings experience a larger radial force stressing the balls of the bearings and pulley shafts. This causes extra friction for the stepper motor to overcome (as the friction force, tangential, is related to the radial force); this means that the stepper has to work harder and can skip steps (for more insight please read below).



While ball bearings are used to reduce friction (opposed to a bush bearing), each ball has a little friction from a couple of sources according to this reference:


  The sources of this friction are: slight deformation of the rolling elements and raceways under load, sliding friction of the rolling elements against the cage and guiding surfaces. 


These effects are generally captured in a single friction coefficient called "&mu;". The relation between friction force (tangential) and bearing loading (radial) is written by $$P_{friction}=P_{load} \times \mu$$ so the higher the belt tension ($P_{load}$), the higher the frictional force ($P_{friction}$), the harder the stepper has to work.