My guess is that the mclarens have worked on the system for so long that they have 3 blown wing setups... with knee and hand , hand only , knee only ...
wild guess though
would have got the idea of using the hand part from other teams ferrari's ... also would have been easy to implement cause most teams have electronics wiring ducts at the side.
from this guess, and the video Lewis uses the hand for the short bursts between turns ... where he cant take his leg of the brake to use it for f-ducting ..
Being a Mac fan .. i dont like it .. Im against f ducks in the first place.