Reply to smart mixing - how to reduce bass whilst moving cross fader

Ive searched the site on this but cant find anything (i tried eq modifiers fader as a search term but didnt find what i was lookig for)

When i press my modifier#1 button i want to reduce the bass on deck A as i bring in deck b.

How do you do this? Ive used modifiers to double up button functions but this seems a little more involved. Ean mentions this is possible in smart mixing blog post but doesnt explain how.

can anyone help?