It's a powerful document and should be tailored to your situation by a qualified attorney. You'll hear two ways of describing this. Typically, this would be when the principal becomes incapacitated, as certified by one or more doctors. With the standing power, the principal gives the agent powers when the document is signed and these powers continue until they are revoked by the principal. A power of attorney is durable if these powers may be exercised even though the principal is disabled.
All powers of attorney expire upon the death of the principal.
An agent simply has the power to act according to the principal's authorization, as set forth in the document. Think of a power of attorney as a second set of keys to your car. Both principal and agent have sets of keys. The principal can have full use and enjoyment of the car and, moreover, can request return of the duplicate set of keys at any time from the agent. Only the Court, through a conservatorship, can take away the principal's rights.