First of all, hi everybody, I believe it's my first post in this forum.
What I have to say is I fully agree with Jack that from a certain skill level on locking your knees together will have a bad effect on your riding esp. concerning shock absorption and reacting to surface changes
BUT
I made the experience that every time I taught basic carving lessons to new carvers I could have told them everything about rotating the upper body, felling compact, shifting the weight and so on but the biggest improvements (esp. with heelside turns) were made when I simply told them "Get those knees together!".
And heellift would certainly help with that.
And that's how I learned it myself, locked knees until I eliminated skidding on the heelside, and then as I became better I gradually "forgot" about this knees thing because it wasn't necessary any more, and at the end I threw away the cant/heellift.
Hmmm, outward cant, interesting idea...