BlackCountryBob
Rookie
Lewis had to save his KERS as he would have lost an extra half a second through the middle and final sectors and lost the benefit of the DRS activating down the straight.
From my recollection of the graphics, Lewis was using about half his KERS after crossing the line and saving the rest to burn out of the last turn where Seb was clearly faster; it wasn't used in the middle sectors.
I guess the compromise was that the entire KERS could have been used after crossing the line to give around 10s of boost but if he did not then make the pass, he would have fallen back badly next time out of the last turn forcing him to have to close back up and maybe not be in a position to capitalise on a mistake.