Your terminology might be throwing your searches. I believe you're talking about "velocity" not aftertouch. Aftertouch is where you control a sound by pressing down harder on the board after you're holding a key. It's commonly used for making vibrato on synth sounds, but almost never used for pianos. Velocity is adjusting the volume of the sound by how hard you hit the key (like on a real piano or most electric pianos). Velocity sensitive keyboards like the Keylab have what are called "velocity curves", which you can choose from that define what velocity the keyboard sends out (this is all midi: 1s and 0s) to whatever synth you're using. You can access this through the Edit > Curve menu on the front of your unit, or through the "MIDI Control Center" software program. The manual has better definitions on which each setting is, as the menu on the unit just gives arbitrary numbers for the different curves. As a piano player, you probably want to try one of the "logarithmic" curves, which means that playing only moderately will generate a bit higher velocity, and sound louder.
As is always the case, be it keyboards, piano, or even drums, you get greater consistency at higher velocity playing, as the instrument can only make "so much volume", so small variations in velocity don't really make much of a difference at the high end. Even on the best keyboard controllers with the best pianists, the vast majority of their performance will be somewhere between 80-127 (maximum) velocity, only settling into the 30-60 range on super-quiet parts. Thus, a "logarithmic" curve will give you the most natural performance, particularly on a light, unweighted controller like the KL61. And yes, one of the "curve" selections is a "Maximum Velocity", which just sends out value 127 (maximum), no matter how hard you play.
