At least on the 66 SL there are 4 air springs.

bottom out
right leg chamber

From what I understand, Marzocchi made a special air system that will never fail you on a steep section and send you over the bars.

First is the obvious one. If the left leg air spring totaly goes out, then you still have air in the right leg to keep the fork up somewhat. At least it can be inflated enough to get you home, harshly, but home.

Second is the primary system for keeping the fork from compressing too much. If the positive chamber gives out, there is still the bottom out progression chamber to hold the fork from bottom travel.

Third is a little less obvious. The negative spring can hold upwards of 200 psi. The positive chamber holds 65 psi or less. If the internal seals inside the air spring give out the high pressure air from the negative chamber will spill out into the positve chamber, making the fork super harsh, but at least UP in its travel as apposed to DOWN in its travel. This can make the difference between a lousy day with a blown shock, or a trip over the handle bars because the fork collapsed and never sprung back.

I feel REALLY confident in hucking this fork. I plan on hitting some nasty drops, stumps, and logs with this beast!