When do my guardians for Medical Powers of Attorney act on my behalf?

If a person has the capacity (i.e. is aware and has an understanding of the things happening around them) then no one can make medical decisions for them.

To put it the other way, if you are appointed as a guardian, you can only make medical decisions for the principal when they lose capacity. This is either through permanent loss of capacity i.e. dementia, or temporary loss of capacity i.e. when they are in an operation and unconscious and the doctor needs a decision about treatment