The Oxford dictionary gives two definitions of "stress":-
1 Pressure or tension exerted on a material object:
2 A state of mental or emotional strain or tension resulting from adverse or demanding circumstances:
From the above it is apparent that the word "stress" was originally an engineering term which has come to be applied to an "emotional" condition.
In purely engineering terms, stress can be removed, reduced or relieved (by providing additional support). However, if engineering stress is "released", it could be because the cause of the stress has been removed (or taken over by another physical means of support) or the stressed object has collapsed.
Extending this by analogy to the emotional state, "releasing stress" would mean that the cause of the (emotional) stress has been removed or taken over by something or someone else or that the stressed person has decided to either "opt out" (let go) of the stressing situation - or to find another way to deal with the cause of the stress.