Decay Setting of a stepper motor driver
It think it would take several pages to explain everything about decay modes in that driver. But the data sheet of the Toshiba TB6560 is available from some suppliers (www.mouser.com). It does a decent job of explaining what the settings does.  

In short:

0% Slow mode

25% &amp; 50% are Mixed decay timing

100% Fast mode

This sets how the driver chip handles back EMF from the motor. It could be useful when microstepping a motor. Fast mode usually gives more current ripple.