In regeneration the motor is actively slowing down the speed (creating negative torque) by pushing energy into the battery. This is achieved in software by regulating a "negative" current, which causes the inverter to produce a voltage that is lower on average than the back EMF generated by the motor. Because the motor is effectively at a higher voltage than the inverter in this condition, current will flow from high voltage to low voltage and therefore flows back through the MOSFETs (and MOSFET body diodes) into the battery.
This is a bit of an over-simplification as brushless DC motors are actually excited by an AC waveform so the current is constantly reversing each half cycle, but the principle remains the same.
As a clarification -- is "back EMF" related to the open-circuit voltage that the motor generates when it's forcibly spun?
In principle it works by just putting a high charge current capable battery on the DC bus, and simply dumping the regeneration current onto the bus, and the battery will eat it, as the bus voltage rises.
The "Architecture" heading, a bit down on this page explains more: https://hackaday.io/project/11583-odrive-high-performance-mo...
Do you know how this (dumping regenerative current on the supply bus) would work if there are multiple motor channels on the same bus that are simultaneously regenerating at the same time?