It is in Chapter Nineteen that Jonas learns the true nature of the word "release" in his community. You would do well to re-read this chapter and consider how the author reveals both to us and to Jonas what actually happens when somebody is "released." Key to note is how this chapter completely transforms the understanding and relationship of Jonas to his community and to himself, and above all to his father. Note how he starts off ironically desiring to see the release of the twin that his father was going to perform that day:
He liked the thought of seeing his father perform the ceremony, and making the little twin clean and comfy. His father was such a gentle man.
However, the reality of "release" is somewhat different. Jonas' father takes a syringe and a needle and fills it with a liquid. Then he injects it into the baby's forehead. As Jonas watches, he sees the baby twin jerk his arms and legs. Then he goes limp. With a sickening realisation, Jonas realises that "release" is actually a euphemism for execution, and he has just seen his father cheerfully perform his duties as an executioner.