Understanding Canon "IS Mode 3"

[koolkurkle]

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?

Thanks

[Marsu42]

I am trying to better understand IS Mode 3 in the new 300mm/400mm 2.8L IS II lenses:

Obviously "mode 3" only activates IS when really taking a picture - which is what I'd like to have on my 70-300L, too, because of exactly the things the manual states: a) it saves battery and skips the annoying IS-sound when pressing half shutter, b) tracking is easier: IS stabilizes the picture for static framing, but "skips" when really wanting to move the lens.

It would be interesting to know how much lag mode 3 produces from full shutter press to picture taken.