Hello,
You've found the right setting -- just keep in mind that it is in milliseconds (so a setting of 1000 = a backstep of just 1 second). This allows you to have very fine control on the backstep time, but it means if you're putting in low numbers there, it might seem like the setting isn't working because its only backing the audio up by a tiny amount.
If you're still having trouble with it after trying some larger numbers, please know that you can contact our technical support by submitting a ticket through this form: http://www.nch.com.au/support/index.html