From Tognar (http://www.tognar.com/waxing_tips_ski_snowboard.html
"P-TEX WAX ABSORPTION
For years, most folks in this industry (like us) have told poor unsuspecting blokes (like you) that sintered p-tex has pores or holes in it...and that, when you hot-wax a base, these cavities sorta suck-in and store wax better than regular extruded p-tex bases. Well, we lied...sorry! But we're not totally off-base (pardon the pun), because sintered p-tex does absorb and hold more wax than an extruded base by absorbing it.
Here's what really happens according to Mr. Urs Geissbühler at IMS (a large producer of p-tex) in Switzerland. P-tex is like a super-cooled, fluid-like substance. It's made up mostly of two different types of microscopic regions...tiny little freckles all packed together, if you will. One type is crystalline and the other is amorphous. The crystalline freckles are, as the term implies, hard crystallized regions that don't absorb hardly nothin'. The amorphous freckles are softer, more gel-like, and absorbent. Sintered bases are comprised of about 60% amorphous freckles and 40% crystalline (and a bit less of both of these if graphite particles have been mixed in). Extruded p-tex is just the opposite...60% crystalline and 40% amorphous.
When wax is applied to the base and heated in, it blends into the amorphous freckles... sorta like sugar being mixed into hot coffee. Then, when the base and wax cool again, about half this wax is expelled from the amorphous freckles, like sugar recrystallizing in cold coffee. This is why waxed bases should be allowed to cool thoroughly before scraping and brushing excess wax off.
Incidentally, sintered p-tex is used on about half the skis made today, but on only 10-15% of snowboards (carving boards mostly). Extruded p-tex is used for most snowboards because it costs less, plus has better clarity so base graphics can be seen more clearly.
P-tex is made in different hardnesses, and both are utilized on racing skis. A hard base is usually used in cold conditions because it can better hold up to the greater abrasion of the cold snow crystals. It also absorbs less wax than a softer base, but this is not so important in cold conditions since there usually isn't as much moisture in the snow. Softer bases are preferred for warmer conditions because snow abrasion is usually less, plus the base can absorb more wax...especially important since wax makes p-tex more hydrophobic (waterproof). This reduces the surface tension of water that's usually abundantly present in warmer snow conditions.
Pete Patterson, formerly a downhill racer on the U.S. Ski Team, does wax testing for Swix at the Olympics and World Cup races. To select which skis to use for wax testing, he starts out with about a dozen pair of Salomon skis (usually womens DH models because they're an easier size for Pete to travel with). He waxes them all identically, then skis each pair through the same speed trap. He then selects 5 pair that test at about the same speed (a .1 second variance or so, anyway), and uses these to do Swix's wax testing prior to DH races. "