OK. Taken from the following:
http://books.google.fr/books?id=7SlKAAAAcAAJ&pg=PA43&lpg=PA43&dq=bisaigu%C3%AB&source=bl&ots=QZe2bTSAGw&sig=idUE7Kd27OXIFgC-UEjSwA7Dpnk&hl=fr&sa=X&ei=R4saUdOFCMWAhQea0oDgBQ&ved=0CEwQ6AEwBDgK#v=onepage&q=bisaigu%C3%AB&f=false
Which is a book about carpentry written in 1857. I've included the original text for those that can read french. The translation is mine. Comments in [] are my explanations.
"Enfin si le charpentier se sert du bédane pour faire quelque entaille creuse comme celle d'une mortaise, le biseau doit se trouver du coté du copeau qu'il veut enlever, et le tranchant doit être perpendiculaire au fil du bois. Pour enlever le copeau après la première incision, il faut en faire une nouvelle qui détermine sa longueur et le soulève par l'effet du glissement du tranchant sur son biseau, de la même manière qu'avec le bédàne ordinaire.
On voit que la bisaigue tient lieu à la fois du ciseau et du bédàne, et même du maillet; son poids et l'impulsion qu'on lui donne remplacent la percussion; et comme le bois qu'on travaille est peu élevé sur le sol, la longueur de l'outil fait que le charpentier n'a pas besion de se baisser. Ceux qui n'ont pas l'habitude de se servir de cet outil, sont privaés de cet avantage, ils sont bligés de se courber et même de se mettre à genous, ou de s'asseoir sur les pièces pour agir avec le ciseau et le bédàne ordinaires en s'aideant du maillet, qui exige un mouvement du bras droit, très fatiguant."
If the carpenter uses the mortising chisel ["bédàne" is a mortising chisel, "ciseau" is a paring chisel] to make a hole like that of a mortise, the bevel must be where the chips will be lifted out [ie if making a mortise in the side facing up, the bevel would be up], and the cutting edge must be perpendicular to the grain of the wood. To lift out the chips from the first cut, he must make a second cut which will determine their length, and lift them out from the effect of pushing the bevel through, in the same way as an ordinary mortising chisel.
We see that the bisaiguë works as both a mortising and paring chisel, and even as a mallet; it's weight and the impulse that one gives it replaces the percussion; and since the wood that on works is not raised far from the ground, the length of the tool means that the carpenter has no need to lower himself. Those who do not have the habit of working with this tool, are deprived of this advantage, they are obliged to bend and even to kneel, or to sit on the pieces to work with the ordinary paring and mortising chisels, using a mallet which requires movement of the right arm, very tiring.
This description suggests that the bisaiguë is actually used as a mortising tool and not just for chip clearance, and also that it is used with a pushing action and not a chopping one (pushed, not thrown).
This user on YouTube (
https://www.youtube.com/user/phojoo) has a large number of videos featuring traditional french carpentry techniques, and while I've not trawled through all of them, the demonstrated uses of the bisaiguë are all using the paring end, either for tidying up mortises, or in some cases, planing a complete surface.
I'll soon be presenting my candidature for a 2 year apprenticeship in carpentry here. If I get the place, I'll be sure to quiz all and sundry about this weird tool. Clearly we've deviated somewhat from Don's original post. Apologies for the derail.
P.S. bisaiguë comes from bi-aigué, meaning literally twin-beveled.
