Click and press the physical button on your controller. Traktor will instantly recognize the MIDI message.
If you use Remix Decks, map Volume > Gain but set the Assignment to Remix Deck > Slot 1-4 . This gives you independent volume faders for the four sample slots inside a single deck.
Traktor has two FX paradigms:
Action only stays active while the button is pressed (best for Cues or Flux).