After the egg opens, the Facehugger emerges and seeks out a nearby life form, latching onto its face, wrapping its tail around its airway, and implanting the Xenomorph embryo(s) by sticking a tube down the host's throat. It is seen sometimes when a facehugger is contained in water or lunging at a soon to be host.
It will coil its tail tighter around the airway to prevent premature removal, and an attempt to sever the fingers found that Facehuggers possess the same acidic blood as a Xenomorph. The Facehugger supplies oxygen to keep the host alive during the impregnating process. Once the embryo(s) has been implanted, the Facehugger falls off, curls up, and dies.
Hosts usually fall into a short coma and wake up ravenously hungry. This can be explained as the embryo needing an immediate source of nutrients needed for its progression into the next stage of its life. Hours might pass between the host waking and their time of death. Predators live the longest as a result of an extended incubation period. The reason for this is likely due to the genetic complexity of the host. The incubating chestburster requires more time for mitosis and cytokinesis due to the morphology of the Predalien.
However, in some cases, the hosts will awake not gradually but suddenly - just in time to be conscious for the Chestburster's push out into the world. Their state of normalcy will last only seconds or minutes.
It was also proven - in the movie Alien that when a host awakens, there is some sort of a memory lapse before the Facehugger actually attaches itself to the host; they don't remember anything that has happened to them. This short term memory loss is most likely a result of the temporary oxygen deprivation caused from the Facehugger's tail, as it asphyxiates the host.