A Posse Commodata happens when one djinn passes his or her djinn power to another djinn by summoning their internal heat and breathing it into the other's ear. It takes around a minute to complete and usually requires a great deal of trust for both djinn.
John gave Philippa his djinn power when they traveled to rescue Faustina. Notably it makes a djinn much weaker, almost like a mundane. To a djinn, who usually feels quite strong, the feeling can be traumatic; a feeling as painful as losing a limb.