...note on projected spark plugs...
One very useful variation of the standard spark plug has its insulator nose and electrodes extended from its metal shell. The projected-nose configuration moves the spark gap a bit farther into the combustion chamber, which tends to improve efficiency by shortening the distance traveled by the flame front and also making the combustion process more regular. But there is a more important benefit: the projected-nose plug provides, in many engines, what effectively is a broader heat range than you get with the conventional flush-nose type. The projected nose is more directly exposed to the fire in the combustion chamber, and quickly comes up to a temperature high enough to burn away fouling deposits after ignition occurs. Then during the subsequent intake phase this plug's exposed tip is cooled by the swirling air/fuel mixture. In this fashion the higher temperatures existing at full-throttle operating conditions are to some extent compensated by the greater volume of cooling air, and the net effect is to make the projected-nose plug better able to cope with the conflicting demands of traffic and highway travel.
It should be evident that the projected-nose plug's effectiveness depends on the pattern of incoming mixture flow. Four-stroke engines often have intake ports angled to promote turbulence. If the plug is positioned directly in the path of the intake flow there will be a large amount of heat removed from the plug's tip by this direct air cooling, and that is just what you get in most four-cylinder motorcycle engines. Indeed, any hemi-head four-stroke engine gives its plugs' tips quite a useful blast of cold air during the intake stroke, and we think projected-nose plugs probably should be in wider use in bikes than is the case. Two-stroke engines can benefit from projected-nose plugs' fouling resistance which they get simply through the sheer length of their insulator (it's a long way from the center electrode's tip back up to the metal shell). However, the two-stroke's incoming charge doesn't always do a good job of cooling its plug, and you have to be very cautious in using projected-nose plugs in the valveless wonders.
Some four-stroke hemi-head engines' domed pistons extend up into the combustion chamber too far, at TDC, to leave room for plug tips that extend inward. This can prevent the use of projected-nose plugs; it's something you check by covering the plug nose with modeling clay, shaping it so you have a 360-degree electrode contour, and inspecting for signs of contact after you've installed your "clearance" plug and cranked the engine over a couple of turns.
Courtesy of :
http://www.strappe.com/plugs.html
I guess "fully projected" means that bad boy sits in the chamber pretty deep.