Same idea as the thionyl chloride reaction. P4O10 is a powerful dehydrating agent. So, the amide oxygen attacks phosphorus to make an imidate. There is a proton transfer step, and then the remaining lone pair on the nitrogen moves down to eject the oxygen and form a protonated nitrile. Another proton transfer step and your done. Basically, P4O10 wants to end up as phosphoric acid, H3[/sup]PO4.