I have tried this myself with 'big butt' mono leaders to soften them up, theory is that once softened the leader material will be more supple, the leader will land straight and the 'cooking' process should not harm the material (ie. weaken it).
had been 'cooked' in the "chaudière" (that the correct word Marc 
).
yup, boiled !
as Hamish notes, the boiling will have a greater effect on the bigger/stiffer parts of the leader but its equally valid for tippet as it makes it all more supple- much better for drag-free drifts.
they also cast better (leader loop propagation) than stiffer mono.
chaudière means boiler room as in heating a house or running a factory. a simple pan will suffice for our needs...
other advantages,
treated materials make for better/easier/tighter seated knots. ill-seated knots are the source of most knot failures.
the material stretches more = less break-offs on sharp head-shakes. an indirect effect of this is that it makes smaller diameter tippets relatively 'stronger' than their original rating.
it slightly gets rid of the top-coat shine most monos have- less fish scary.
what this also does is make the coating hold sink paste easier and for a longer period, the biggest issue i have with high-end contemporary monos is their coating doesn't let it hold easily.
one can also lightly tint the mono to suit water colour. a favourite for stained (peat-stained or whatnot) waters is adding some coffee grinds to the water
how- boil water in a pan, bring down heat to a simmer and put in your mono for 3 minutes. pull out with a fork, end of story.
hard to think of something as easy that's so beneficial.
typically the biggest worry is that this process weakens/damages the mono. it doesn't.
boiled water is at 100° whereas many times more that (from what i hear, manufacturers don't want to give exact figures) when the mono itself was originally made.
cheers,
marc
