This is a follow up to my previous question: Plain English Meaning Of Trust Language, which I included a picture of for clarity. The trust article outlines what happens if an heir is deceased but what would happen if one of the successors chose to forfeit their share of the trust? Would that same hierarchy of distribution still apply, allowing their share to pass to their children instead of themselves or being that it is not specifically mentioned, would forfeiting their share be the same as severing their ancestral branch from the trust all together?
I think that you are asking about a "disclaimer" which is a formal refusal to accept a donative transfer.
In the absence of express language in a trust stating what happens where there is a "disclaimer" (which is not unusual to find, there are so called "disclaimer trusts" that specifically have such terms), a disclaiming party is treated as pre-deceased for purposes of the trust if the disclaimer is made in a timely fashion and in the proper manner.