Well, it's a relatively simple question to answer, perfect for my limited knowledge, specially because the regulations are clear:
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Confor foam
This is a trademarked foam, quite particular, as most materials,
according to manufacturer EAR "would amplify the rebound significantly after head impact, or they would bottom out, or collapse completely, offering little physical protection".
This foam hardens when impacted, as krisfx mentions. It also rebounds little.
This is the article by Scarbs that krisfx talks about:
http://scarbsf1.com/blog1/2013/01/16/in ... ead-rests/
There are three differente grades of foam (instead of the two krisfx mentions) depending on ambient temperature, at least according to FIA.
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Soft" carbon fibre cover
The foam is covered with carbon fiber for aerodynamic purposes. As you can imagine, regular hard carbon fiber would negate the impact adsorbing effects of the Confor foam. Hence you have to use a thin cover made of carbon fiber that is soft and almost, according to Scarbs, "rubber like".
You can read Scarbs article to find the curious ways in which teams go around the rules for less aerodynamic drag.
There is also a nice rather old booklet I read by Sid Walkins, called Walking Away (with one of the most spectacular pictures of an accident I've ever seen in a booklet about safety, if you ask me).
FIA's idea seems to be for everything to be reduced to pieces, except the driver (and perhaps the tyres)
I mention this booklet because it explains the curious ways in which this technology has moved into rally racing seats.
Same headrest idea, but only made of foam: if you're going to learn how to do headrests, start with these
