Hand of Ipecac
This phrase refers to Sam's power as first seen in 4.01 Lazarus Rising to expel demons from possessed humans and send them back to Hell. When he does this it appears the possessed human is vomiting out the demonic smoke. Ipecac is a solution used to induce vomiting - hence the term.
The term gained wider currency when it was mentioned to Jared at the Salute to Supernatural Chicago 2008 convention.