I don’t think there is a firm consensus. I have boiled the common F-A question down to the info below:
F-A Pros:
- This motion can improve comfort in the saddle. Some people claim this has more benefit than the Left-Right Rocking motion.
- This motion can improve the stability and feel during transitions between seated and standing positions. It adapts to the shift in center of mass that we experience during these large body movements.
- This motion can improve the feel of standing efforts at regular power outputs. It adapts to the shift in center of mass that we experience during these large body movements.
- This motion can also improve feel during all-out sprint efforts.
- A rocker without this motion can wiggle around on the floor during these big efforts, while rockers with this motion are more stationary and stable on the floor.
F-A Cons:
- This motion typically requires more parts and complexity to allow and control the related motion along with the regular rocking motion. Rockers with this motion are typically more expensive as a result.
- This motion can feel odd at certain powers and cadences. Much of this depends on the centering forces related to the motion, rider weight, pedaling style and other factors.
- This motion may even lead to power loss as the bike is moving in ways that counter the power production cycle while pedaling. But this is far from settled and experiences vary widely.