I am trying to better understand IS Mode 3 in the new 300mm/400mm 2.8L IS II lenses:
From Canon Marketing
"A third Image Stabilization mode (Mode 3) activates IS only when the shutter button is fully pressed, and gives the equivalent effect of a shutter speed four stops faster"
and from the Canon Manual
"MODE 3 - Since camera shake is only stabilized during exposure, following a subject is easier such as when shooting a fast and irregularly moving player during sports photography"
How does Mode 3 make following the subject easier compared to Mode 2?
Since Mode 3 saves battery, why would you ever use Mode 2 with these lenses?
Is the 4 stops claim true for all modes, or does Mode 3 give an effective increase over Mode 2?