Don't mix it up with random for this drill mate , save the random stuff for the more regular 4 corner drills or random arms.
This ones pretty much about just improving your raw hand speed and then attempting to control his arms when you can't keep up with his hands any more.
Your stock standard 4 corner drill pretty much goes , he punches , you do your Dai Sau and punch or your Chit Sau and punch , at normal speed there will be a slight pause before he punches again allowing you to comfortably complete your counter attack with both hands.
In this drill he starts off normally , LEFT HIGH , RIGHT HIGH , THEN LEFT LOW , RIGHT LOW with big looping punches then he goes faster and faster and gradually bringing his punching trajectories in tighter and tighter.
( the attacker still keeps the same sequence left , right ,left , right it just becomes a faster and tighter circle , till the punches all seem to be coming at once.)
Pretty soon you find yourself struggling to keep up the pace with the normal Dai Sau / Jit Sau counter attacks , its starting to get very messy and your in danger of being hit so you must go forward using your Chi Sau to try and get inside his arms , stick to and control both of them and then strike through.
I suppose you could say that at first we are using our visually based reflexes and once they start to fail because the action is becoming way to fast , then we must move forward and gain contact to rely on our touch based reflexes.