How are "sink back" and "sink into" different?

Rebecca

"Sink back" means to lean back and relax into something, like a comfortable chair. "Sink into" means to gradually go deeper or penetrate into something, like a liquid into a material. When we are referring to the action of sitting, usually "sink back" is used. Ex: He sank back into his chair. Ex: The dye needs to sink into the cloth.