Det verkar som om John har
transformed one finger into a blade in order to stab Sarah in the shoulder. (We can see the wound on Sarah's shoulder in later scenes.) This is somewhat different from the way the Skynet Terminator transforms John; the Terminator presses its hand into John's face and an orange glow appears around its fingers.
Detta har effekten av
immobilising and weakening her, and distracting Reese and Pops. As John explains, he can't kill Sarah, because if he does, Pops will be free to detonate the explosives and destroy them all. But he certainly can hurt her. (This is also a probable homage to a scene at the end of Terminator 2, in which the T-1000 similarly stabs Sarah in the shoulder.)
I senare scener
Sarah does not appear any different, so it seems John was unwilling or unable to transform her into a Terminator like himself.
Med detta sagt, telegrafiserar slutet av filmen ganska blatant att det kommer att bli en uppföljare, så den aktuella händelsen kan ha någon sorts försenad effekt på Sarah.