I would invite you to take a look at the H-Bridge IC datasheet, most notably the schematics and diagrams on page 4 of the PDF.
Hard braking generates a “wheels locked” condition on the H-bridge. This amounts to the H-bridge being configured in such a way that the H-Bridge outputs are both driven low. They refer to this as a “Brake” condition in the datasheet. A DC motor shaft will not turn in this condition, and it resists external attempts to force it to turn.
Soft braking generates a “free wheeling” condition on the H-bridge. This amounts to the motor being disconnected from Power and GND altogether. Stated differently, the H-bridge is configured in such a way that the H-Bridge outputs are both high-impedance. They refer to this as a “Stop” condition in the datasheet. A DC motor shaft will freely rotate if external forces are applied to it.