uncinches
Undoes the belt
To uncinch is to loosen or release a cinch—the strap that secures a saddle around a horse's belly.
By extension, uncinching applies to loosening any tight fastening: She uncinched her belt after the big meal.
The word carries a sense of releasing tension or pressure.
slacken
Become less taut
unlace · unhook · detach · disengage · unbridle · unhitch · unclasp · unpin · unsnap · unlatch
girth
Measurement around something
halter · harness · stirrups · reins · martingale · bit · pommel · cantle · mounting · dismounting
tension
Stretched or strained state
decompress · unwind · let go · breathe · loosen up · de-stress · unclench · soften · yield · surrender