Decay Modes - Stepper Motor related Query
Remember that current and voltage are always tied unless there is an impedance in between. That impedance could be (and often just is) the wiring but usually we think at components as 'directly connected'. In the same way 'instantaneously' doesn't exist, otherwise mechanically the motor would rip itself from the bearing from the inertia and torque.
Decay modes in stepper are mostly related to how current is regulated in the winding. Do not confuse steppers with brushed DC: a supplied DC motor runs, a supplied stepper motor just holds position with some torque. You need to commutate the phases to do this.
Also back-EMF is way less relevant in stepper unless at high speeds (it's used for stall detection, for example, but it doesn't work at slow speed).
Since current regulation in the winding is done following the current ramp of the winding decay modes changes indirectly the control profile: the driver can only make the current rise (with the time constant of the windings) or let the current decay (either shorting or leaving open the bridge). The resulting current ripple is actually torque ripple (holding torque or running torque depending on the mechanical alignment).
The issue arises especially with microstepping when the current controlled is different from the current one: it can only rise at one speed but can decay at two different speeds. It gets even more complex since the drive can use fast decay for one part of the cycle and slow decay for the rest of the cycle. Like for PWM the net result is any decay speed in between (this is called mixed decay).
Each controller does it in its own way: I used the DRV8885 and there are more than 4 pages in the datasheet explaining pros and cons of each decay modes.