1- If the stabber is executing a proper straight stab, you dont have the time to move your feet before the block In fact you want to refrain teaching moving your feet before the block. You have to adjust your stance by bending your knees if the attacker dips for a low stab. Unless you are standing facing the live side. Then your foot will be in the air before the stab because you are pushing your belly backward to avoid the blade.
That alone will not let you move too far from the attacker so it also hels control his possible second attack as you follow his retracting arm with a leap forward landing with yoru foot behind his front foot. During that leap you want to maintain pressure at the end of your thumoutward pivot keeping the elbow below his arm not letting it escape and presuring forward not letting him cut your belly.