In light of the Brigham City Temple dedication service coming up this Sunday, I thought I'd do a little reading up on the Hosanna Shout. In my life, I've only participated in the Hosanna Shout a few times as it's quite the rare occasion that permits its use.
The Hosanna Shout has its roots in the Old Testament. The ancient Feast of the Tabernacles consisted of a choir that sang psalms including phrases similar to our "Hosanna, Hosanna, Hosanna, to God and the Lamb!" Rather than waving white handkerchiefs as we do now, they waved palm branches. The symbolism is the same. It's a plea for peace and a prayer for salvation - particularly to send the Savior. Anciently, a priest would pour water from a golden pitcher as the gathered waved their branches to the psalm. The picture of water represented the sending of the Savior from heaven to earth. [Old Testament Student Manual]