Lead Flexing in 2-Person

[gojeremy]

I have been doing a lot of 2 person this year, and my instructors/clinicians have advised to flex over to ball side when as Lead. I have been told to wait until ball goes into lower corner as a good indicator as when to flex over. I am having a bit of trouble with this, because when I flex, that is when the ball starts to rotate to top of the key. Do I immediately flex back? Should I be flexing sooner before ball goes to corner? I have been working with a lot of veteran officials who say there is no need to flex and I should stay opposite trail at all times and have the players boxed in. Advice??