I just put a 2003 Z1 Freeride on my bike and it a great fork. The best version Zoch has made yet. The ETA makes climbing so much better.
Here is an explanation right from the Marzocchi site,
ETA EXTENSION TRAVEL ADJUST
When Marzocchi intoduced the ECC system, we were the first to offer an easy travel adjust function.
This function reduces the forkís axle to crown length making steep climbs and switch backs a whole lot easier.
Since itís introduction, other manufacturers have jumped on the shuttle and introduced their own versions of travel adjustability attempting to provide the ideal travel adjustment system.
Our designs are built with one thought in mind, pure functional performance that benefits each and every rider. Our ECC and ETA systems are topped mounted and can easily be adjusted by a flick of your finger.
If your favorite trail takes you on an undulating fire road or single track youíll know that stopping at the top of every downhill to adjust your fork is out of the question. Not to mention if your riding partners are going to wait for you.
The new ETA system provides the same ECC performance advantage along with 30mmís of active travel no matter where the fork lockís down. To explain it another way, when the ETA is turned on, the forkís rebound is locked-out as in the ECC design and the suspension will compress and settle into itís travel based on riders weight and terrain. However, instead of the fork being completely locked-down and rigid, there is a 30mm long spring at the bottom of the ECC cartridge delivering 30mm of active travel to take the edge off small bumps and rocks.
This design is only available in coil spring systems and is used in conjunction with air systems (one side air and one side ETA coil).
That should answer your question.