Without getting into a long and technical discussion, for these, engine braking is just the use of the engine slowing down to slow the vehicle. As with any, that ability is increased in lower gear ratios so as the CVT back or downshifts on decel back to lower ratios, it take advantage of that resistance offered by the engine. That is.. until the primary return spring overcomes the centrifugal forces presented by the flyweights. At that point the sheaves release from the belt and the belt rests quietly on that idler bearing. This could be added to our discusion on the engagement as a lower engagement also means a lower disengagement on engine braking. For example right now you might have engine braking all the way down to say...8 or 9 mph (just guessing) where a spring that offers less resistance to the flyweights would also allow that to drop to say 4 or so mph. My old Kawasaki had a electronic mechanism that held the primary sheave against the belt until 3mph then would release. This was great but without that equipment it would release closer to 9mph with the springs they used. As for the secondary, it's spring preload and helix ramp angles offer changes in not only shiftout but backshifting. You could say increasing the spring preload will cause increased resistance to the primary's flyweight forces and force it to be in a higher RPM range but that same force will also force it to backshift harder which puts it in lower gear ratios as it decelerates, making the engine braking even more pronounced. A bit of a ramble...sorry. But I hope that helped.