From ebay auction by bicyclists_retreat:
This offering is for one set of new-old-stock Campagnolo Athena Syncro (7-speed) shifters with a complete set of original hardware and cables to properly mount these shifters onto down tube shifter bosses. The Syncro design represents Campy's first generation index shifting technology from the late 80's and very early 90's. On balance, however, we believe the product/technology never really lived up to it's lofty expectations. By all accounts, the workmanship and production was typical Campy quality, but the basic design ideas really over-reached the boundries of what was realistically possible at the time. More specifically (and as noted in the Syncro brochure that accompanies each shifter set), Campy set out to create a shifter set that was backwards compatible with older non-index shifting Campy drivetrains (as well as other non-Campy drivetrains).
From what we can understand, the idea of cable travel represented the most significant problem in developing index shifters that touted such broad/backwards compatibility characteristics. Cable travel is nothing more then the cable movement needed to shift a derailleur from one gear/sprocket to an adjacent gear/sprocket. In the case of vintage Campagnolo derailleurs, very short cable travel was all that was needed to shift between sprockets. This characteristic worked fine in the frinction-shifting environment where fine shifter adjustment (a.k.a trimming) was possible. However, with the advent of index shifting, the trimming function became extremely limited (and non-existent in many cases). This presented a problem for Campagnolo, because they developed their Syncro technology to accomodate this short cable travel, which generally places more tension on the cable (relative to longer cable travel), increases cable friction and the likelihood for cable stretch. These shifting environment characteristics, coupled with little or no trimming functionality, meant that drivetrain precision had to be extremely high to achieve consistent and functional index shifting. Not suprisingly, this high hurdle of having all drivetrain components installed and adjusted to work within such fine tolerances proved to not be very practical at the time. As a result, index shifting was less than ideal on many drivetrains with Campy's Syncro shifters, which forced Campy to discontinue it's Syncro products in the early 90's (in favor of newer index shifting technology introduced shortly thereafter...with longer cable travel...that has performed much better).