hello
how do you use hubris - i confess it has me a little stumped as to how what and why it can be used?

The main 'problem' i have it that if a character fails a simple contest (e.g. sneak into a fortress without being seen...) he can then declare hubris and turn it into a 'battle' -- how does that work?!

Any sage advice appreciated

Very simply - the player decides not to accept the failure and goes for the battle (extended contest).  In narrative terms you've gone from a scene cut (Achilles announces "I shall sneak into the fortress" cut to (success) Achilles peers out from behind a statue in the great hall or (failure) Achilles is dragged by guards before the tyrant) to an extended scene where you are playing out the cat and mouse events of sneaking into the fortress.

As mentioned on page 36, if a player decides to invoke Hubris, and they lost the initial contest, they immediately take a wound for each victory the enemy had over them in the simple contest.  Narratively, this turns into the first setback trying to sneak in ... a guard becomes suspicious, the sea tunnel has an unexpected grate, and the tide is coming in, etc.  Then set the battle dice as described for non-combat battles on page 60.  The position and attack roles should be narrated in terms of the type of struggle being carried out, e.g. a guard attacks by peering over the battlements while Achilles responds by hanging from the wall out of sight, etc.  A battle in this context does not have to mean that blows are exchanged, simply that you are resolving events in a detailed action by action manner.

Any of this helpful to you?

excellent response - many thanks indeed

upon further reflection I am tempted to limit invoking hubris to once per objective or maybe once per quest - I am wary of how much work this ability could put on an antagonist when a player invokeshubris because he failed at a simple contest and wants another go albeit starting with some disadvantages and maybe even wounds - could slow a game right down is the danger here

Think of Hubris as a throttle for the players. Sometimes, they get to pace the game, instead of the GM.

It also lets the GM collapse "big" stuff into simple rolls, if she likes, because the players always have a safety valve if they want it.

GM: "The kraken rises from the deep, and with a shrug of its massive bulk, swamps the seaside village under a giant wave, killing the local fishermen."
Players: "No way! We save them. We run ahead of the wave and get people to safety."
GM: "Okay, Athletics contest."

That's one contest to kill or save a whole village! Nifty. If the players lose, and they care a whole lot about the village, they can use Hubris and burn more resources in a full-on battle.