Compared to most existing systems out there SCD provides a difference in the portion of travel from the fully extended position to the sag point. This is the portion of travel used when you sit on the bike, and what happens in this part of the travel is critical to bike performance when pedaling over bumps.
The response to pedaling forces above the sag point allows the suspension system to return to the sag point. (Traditionally other systems provide maximum anti squat when the suspension is fully extended which can unnecessarily hold up or extend the suspension.)
SCD provides reduced anti squat (below 100%) when the suspension is extended above the sag point:
The benefits of these outcomes are explained here.
The goal with SCD is to compliment the slightly lower anti squat above the sag point with a tuned higher anti squat (e.g. around 100% or slightly above) when the system is around and compressed past the sag point. This provides for efficient pedaling and a firm feel under hard pedaling.
Test Proven Performance Benefits
Test results support the benefits of SCD when pedalling over obstacles and up hills. Go here to see the results.
Vast was originally conceived around a simple concept. Reduce the anti squat above the sag point to focus the system dynamics towards the sag point. In its original form, this outcome was achieved via a new linkage layout that provided a wheel path roughly representing an "S" shape. However, following further analysis, development and testing, this original concept has been developed much further. Significant reductions in anti-squat above the sag point below 0% are not really needed. A more refined approach is to provide an anti squat curve that is more consistent and changes less rapidly.
This is now provided by the second generation of VAST designs. Refer below for a comparison of Gen I and Gen II anti squat configurations.