APM BIG-IP as SAML SP - External IDP Single Sign On Service Binding

When you are configuring the BIG-IP as a SAML SP and are configuring the external IDP there is an option for the Single Sign On Service Binding. The default option is "POST" and the other option is "redirect". What is the difference and when do you use one or the other? I have found references in documentation to this configuration item but no real understanding. I'm seeing different behavior when I have POST enabled, but I'd prefer to actually know what is going on and what the difference is rather than just guessing that I should be using "redirect".