The passive braking of the TMC2130 is achieved by using the resistance of the MOSFETs to short out the stepper motor coils. Since there is both N-channel and P-channel MOSFETs used, there is effectively three settings:
1) Freewheel mode - stepper motor coils are open and motor shaft can spin freely
2) Low side braking - stepper motor coils are shorted using low resistance N-channel FETs resulting in heaviest braking
3) High side braking - stepper motor coils are shorted using higher resistance P-channel FETs resulting in modest braking