I was fortunate to have a great Tai chi teacher who taught me many of the ways of the classics. Holding a rod for me was similar to using tai chi whip or tai chi sword. For these weapons a loose two finger grip was advocated. Thumb and index fingers; similar to a badminton raquette. That's not to say just 2 fingers held the rod or sword. But the focus in your mind was in these fingers. A loose relaxed grip was preferred.
The rationale was that a loose grip allowed your weapon or rod to naturally become an extension of your body. No tension or white knuckles. A very loose wrist as well. The sword technique was called sticky sword. Contrary to the hard barbaric clanging of swords that we are used to seeing.
A softer approach was recommended. As well the elbow was always left relaxed and down. By properly weighting your legs (opposite of rod hand) you can swing and turn in the hips to get more power. Hope this makes sense.
I have found that the more tension free my limbs are the better my cast. Developing rod sensitivity is the name of the game.