In answer to the question of what constitutes the handle of handles, we would invite the questioner to, literally, try his hand and apply the “four rules of good grip.”
Thumb Guide
The thumb is always looking for a sense of direction. There is evidence of this on even the earliest man made implements. A great number of hand held objects feature channeling for the thumb of one sort or another.
Forefinger Furrow
The forefinger is likewise in on the orienteering process. It acts as a pilot, paving the way for the rest of the hand to follow. We have located recesses for the forefinger on any number of artifacts.
Palm of the Hand Support
Once the thumb and forefinger have finished their reconnaissance work, the hand as a whole goes into action. The palm of the hand needs to be well supported at this point if sufficient leverage is to be generated.
Gripping Substance
The hand does not like clutching at thin air and is only happy when it can roundly envelop. This is reflected in the way people roll tactile objects about in their hands.