Casting an Expellarmus spell in the middle of an empty coridoor wasn't the brightest of ideas, but it was rather an accident. Just as you consider letting the spell hit you, Snape walks past. Amused, he stops to watch as you desperatley cast a deflection spell... The longer you can deflect your previous spell, the better.

Move your mouse from left to right to stop the spell from disarming you for as long as you can.

Almost smiling, Snape hands you your wand before swooping away.