A Healthcare Proxy is the document in which you appoint a Healthcare Agent to make medical decisions for you when or if you are unable.  This agency can be temporary or permanent, depending upon your capacity. 

For example: if you are in surgery to remove your gallbladder and the surgeon found the need to remove your appendix because you are unable to consent, your Agent will be consulted.  When you are awake and out of surgery, your Agent is no longer needed, as you are capable of making your own healthcare decisions. 

Should you be permanently mentally incapacitated, the doctor may invoke your healthcare proxy, in which case your Healthcare Agent will act for you from that point forward.  This document is necessary as part of your estate plan and it is recommended you provide all of your healthcare professionals with copies.