How Test Cases in Tests field is getting freezed once the Test Session is moved to In Testing state

As per my knowledge Test Cases linked to Test Session in Tests field will not change if the Test Session is moved to In Testing state. That is because TestSession.js trigger sets the Tests As Of date. But I am not sure how Test Cases are prevented from showing the updated information after As of Date. Is there a trigger or in built functionality of PTC?

I am curious to know how this works.

Thanks,

Vikram M