Hi Scott,
There is actually a good help topic for this called 'Setting valve gates'. If you access the help, go to the index tab and start typing 'valve'. You will see 'Setting valve gates' turn up.
The valve gate trigger you are looking for is 'control by flow front', and you can even specify a delay to let the flow front pass sufficiently before opening the gate.
I hope this helps
Hanno van Raalte,
Product Manager - Injection Molding & Moldflow products