Because of his intricate knowledge of the Accelerator's skills, Amata can bypass Accelerator's Redirection. Amata can hit Accelerator by pulling back his attack right before it is redirected by Accelerator's AIM field, because it simply changes the direction of the vector towards its opposite, as such anything that is coming towards Accelerator will be redirected back in the opposite direction. With this method, Amata can use his "Counter" to retract his fist the moment before it makes contact with Accelerator's AIM field, so the Redirection instead pulls it towards Accelerator. 

^ This is what I meant. Basically the shield redirects vectors the opposite direction. So basically if you pull back when the Vector attempts to redirect then it will pull your fist towards the user of the Vector Field instead of push away.