Release is death by lethal injection.  There is a ceremony before the release, but the release itself is just an injection.

When a person becomes old enough, the community has him or her euthanized.  They are not necessarily sick or infirm, they are just no longer useful.  Before the release, there is a telling of the person’s life.  When Jonas volunteers at the House of the Old, he finds out about the ceremony of release for the old.

[The] telling of his life … is always first. Then the toast. We all raised our glasses and cheered. We chanted the anthem. He made a lovely good-bye speech. And several of us made little speeches wishing him well. (ch 4, p. 32)

Then the person walks through a special door to the Releasing Room.  The whole thing is a special occasion, and one of complete happiness.  They do not know what will happen to them until it is too late.

Jonas wonders about release when he realizes that Larissa, an old woman he gave a bath, has been released.  He imagines her life as “quiet and serene” in Elsewhere (ch 14, p. 115).  He does not know what release really means.

Jonas is shocked when he learns what release really is.  He sees the video of his father’s ceremony of release for the newborn infant.

His father turned and opened the cupboard. He took out a syringe and a small bottle. Very carefully he inserted the needle into the bottle and began to fill the syringe with a clear liquid. (ch 19, p. 149)

Jonas is concerned when he finds out what release really means.  He thinks about the ceremony of release he saw for the Old.  Jonas says she loves the old, and worries about how she will feel when she finds out.

"Fiona is already being trained in the fine art of release," The Giver told him. "She's very efficient at her work, your red-haired friend. Feelings are not part of the life she's learned." (ch 20, p. 153)

The people who actually conduct the release have to know that they are killing them.  However, there is no place for feeling in the community. 

Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book) (p. 153). Houghton Mifflin Harcourt. Kindle Edition.