Kytrinh,

Press the key down (as you normally do now to hear a sound). When key gets to the bottom, now press a little harder (you will fill it move even lower but with more pressure applied to get it to do that) - that is AFTERTOUCH. If the FILL TO AFTERTOUCH IS active (or any other program assigned to after touch (e.g. Voice VOLUME/PITCH/BEND ... etc)) you will now 'hear' that effect.

Hope that helped?