@msilvestro:
I agree with Jeff S when he says that everyone has variations of technique, even when they mainly use one style. I personally use three general styles of playing right now, one of which is similar to the knuckle extension technique described above. My current technical struggle involves integrating these three methods. Each has its own advantages, and its own drawbacks. I call these "closed", "medium", and "open" styles.
In my "closed" style, my right hand is mostly closed (I play right-handed) with the thumb and index together, and my other fingers curled with the finger tips pointing towards my palm. This style is very accurate for musical fragments where the notes are close together and/or repeat a pattern several times rapidly. It is also good for transitioning up and down scale patterns, kind of like "crossing over" in piano technique. The disadvantages are in making large leaps, or in doing slow slides across a larger interval. I believe my "closed" variant of playing is similar to the knuckle extension method you are describing.
My "medium" method is a lot like the form demonstrated by Lydia Kavina in her demonstration DVD which was included with my Etherwave. This method also involves holding the thumb and forefinger together with the fingers curled toward my palm, but in this variant, the middle, fourth, and pinky fingers open from completely closed, with the fingers touching the palm, to completely open, with the middle, fourth, and pinky fingers extended to almost straight, pointed towards the pitch antenna. This method has the advantage of allowing larger interval jumps, as well as allowing pitch bends up or down by curling or uncurling the fingers. I usually tune my theremin in the medium method so that I can cover an octave by opening my hand from mostly closed to mostly open. I try to keep my third, fourth, and pinky fingers separated and curved as I open or close my hand, as I find that this gives me the best pitch control.
My "open" method involves holding my hand kind of like a cat flexing its claws. I only use this method sparingly, but it has the advantage of allowing very slow controlled bends up or down. In this method, the thumb and forefinger are not touching, and I open or close my hand as if gripping a ball, again with all fingers curved. This method is not good for doing fast controlled repeated patterns or general playing (at least for me).
I have come to my own style(s) of playing by watching videos of players I admire, and trying to borrow elements of their playing that might work for me. You will have to do the same...This instrument is difficult, wonderful, and demanding. Part of theremin's attraction for me is the challenge of trying to master small elements of playing style. Try as many variants to your playing as you can, so you can find what works for you. The bottom line is whether your method allows you do what you want to do and play the things you want to play.
What Jeff S said about not taking anyone's recommendations too literally of course also applies to what I have just said. Above all, don't let the difficulty of this instrument get you down. Have as much fun as you can.