[ Had he been caught unaware, there's a good chance that heel would've connected but as it is he got ample warning. Vampiric speed and reaction time helps too — he bends easily backwards, leaving the blow hitting nothing but air.
Not that he stops there. Oh no. With help from the momentum in the movement Spike goes low, swiping out his own leg in an attempt to knock her on her ass. Two can play this game. ]