Depress the button on the seat belt buckle.
If the belt does not fully retract, pull it out and check for kinks or twists. Then make sure it remains untwisted as it retracts.

NOTE If a belt does not fully retract, inspect it for kinks and twists. If it is still not retracting properly, have it inspected at an Authorized Mazda Dealer.
Fastening the Seat Belt
Front Shoulder Belt AdjusterJudder Upon Torque Converter Clutch (TCC) Operation [Fw6 A EL]
TROUBLESHOOTING ITEM
Judder upon torque converter clutch (TCC) operation
DESCRIPTION
Vehicle jolts when TCC is engaged.
POSSIBLE CAUSE
Signal malfunction
APP ...
Coupler Component Removal/Installation [Fw6 A EL]
WARNING:
A hot transaxle and ATF can cause severe burns. Turn off the engine and wait
until they are cool.
Always wear protective eye wear when using the air compressor. If the air
compressor is used, any particles of dirt or soiling could spatter and get into
the eyes.
...
Underbody Maintenance
Road chemicals and salt used for ice and
snow removal and solvents used for dust
control may collect on the underbody. If
not removed, they will speed up rusting
and deterioration of such underbody parts
as fuel lines, frame, floor pan, and exhaust
system, even though these parts may be
coate ...