In the panel at page 102 of Blood of Martyrs:

"when a player's character is in a position to burn his final Fate point to save his life, he may instead choose to Martyr himself in service of his Faith."

So i can't understand how i can Martyr myself, because if I am in a position to burn my final Fate point to save my life, it's clear that I am dying;

so if i don't burn this Fate point i'll die and i will not Martyr myself for a number of Turns equal to my WB......

So how does this rule work?

It is effectively 'going out in a blaze of glory'.

Don't worry too much about the how and why, because the rule overides the imminent death. You trade your option of buring a fate point to survive for a mighty last stand. When you are about to die, instead of burning a fate point to survive and if the gm allows you declare that you will martyr yourself.

For a number of rounds equal to your willpower bonus (including the round in which you are 'killed') you can continue to act and ignore further criticals unless that critical takes out a faculty such as a limb, head, or senses. (of course as this includes the round you should have died, if you lost an arm or something then that arm is still gone, you don't die from shock or blood loss until the end but the arm is gone). You also get to use one faith power as if you had burnt your fate point, and at the end you finally die and your buddies all get a spent fate point back.

And your PC becomes legendary.