[i]"not extending my fingers all the way out but rather playing more with my knuckles" - Para..[/i]
LOL - I ma going off now into one of my longwinded technical essays! ;-) Please be warned, and skip this posting if you are an artist who doesnt know what capacitance is! ;-)
I cannot give any advice on playing.. But from a technical perspective, I see little merit in ever fully extending the fingers - this opinion (and it is only an opinion - I could be completely wrong!) comes from crude analysis I have done on capacitive distributions between the players hand (mainly) and the antenna.
As everyone knows, it is the capacitance 'seen' by the antenna which determines pitch.. Greater capacitance = Higher pitch.
There are two main factors that determine capacitance - these are: 1.) Area (The area of adjacency between the two capacitor plates - as in, the 'overlap' area of the antenna and the hand). 2) Distance (the distance between the plates - as in the distance between the hand and the antenna).
I will go straigt to my 'conclusion' first, and follow this with 'theory' next.. I believe it is possible to actually reduce capacitance by straightening the fingers if the decrease in distance of the hand to the antenna is accompanied by a decrease in effective 'overlap' area of the hand to the antenna.. Actual reduction in capacitance is an extreme (and unlikely) case, but what does happen is that the increase in capacitance can be much less than one would expect if you are only thinking about distance and not taking changes of your hands area into account.
I think the above particularly (but not exclusively) applies to players with harge hands and thin fingers - With such a hand extended (flat) pointing at the antenna, the area of the fingers closest to the antenna is small.. but the same hand, bent (using knuckles) represents a larger area and movements of the fingers which change the effective area give (in my opinion) much greater control of the capacitance and therefore pitch.
Everything is actually a whole lot more complex than what I have stated above.. Even plotting capacitive distributions in 2d becomes a mammoth undertaking - One has the 'direct' coupling where the hand and antenna overlap horizontally, and one has all the vertical couplings (which are angular) from the antenna above and below the hand - One has these 'vertical' components extending for the whole length of the arm, each adding to the total capacitance (so changing ones hands distance from the antenna by moving your arm does not only reduce capacitance by merit of the hand <-> antenna distance, it also reduces capacitance through the 'vertical' capacitive components) - and then, of course, there is the body's bulk capacitancewith all its 'direct' and 'angular' couplings to the antenna.
The basic equation for capacitance is C=eA/d where e is the dielectric (assume 1 for air, but this can vary with humidity, temperature etc) A is Area (this is the 'overlapping' Area of the 'plates' - This becomes quite complex when one is looking at hand <-> antenna relationships) and d is distance (again, even this is not a simple matter when one is looking at hand/arm/body relationships with reference to an antenna).
Everything about a players body / position / hand orientation etc will affect the capacitance in complex ways.. For example; A flat horizontal hand, fingers pointed to the antenna, will have little 'common' area between the closest distance and the antenna - however, such a hand will have a lot of coupling between the hand and the antenna above and below the hand (one needs to think 3d to visualize this).. A flat vertical hand, fingers pointed to the antenna, will have more 'common' area between the fingertips and the antenna - however, such a hand will have a less coupling between the hand and the antenna above and below the hand.
I am sure there is more than enough 'meat' in the above topic