The vehicle is being slowed by engine compression.
(Wheel ➞ Engine)
Kia Niro 2017 (DE HEV) Service Manual: Rear Seat Belt Buckle Repair procedures
Replacement
When installing the belt, be careful not to damage the retractor.
1.
Remove the rear seat cushion assembly.
(Refer to Rear Seat - "Rear Seat Assembly")
...
Kia Niro (DE HEV) Owners Manual: Before starting
Close and lock all doors.
Position the seat so that all controls are easily reached.
Adjust the inside and outside rearview mirrors.
Be sure that all lights work.
Check all gauges.
Check the operation of warning lights when the ignition switch is turned
to the ON position.
Release ...