Hi Bob -
I presume what you are experiencing is that the pressure applied with palm-pad at the base of the index finger feels less precarious than the pressure appplied with the thumb up away from the hand. However I would not recommend starting your learning curve with a finger on top (FOT) grip unless you are absolutely sure you will be fishing short casts with easy effort, and typically lighter weights.
My recommendation, FWIW, is to stick with a 'relaxed' thumbs up grip for at least your initial learning curve. For me, when the TU grip is not comfortable it meant I was not doing something right. Now it's perfectly comfortable.
When learning the objective is to hold and move the rod so that it can flex and recoil in the proper cadence, not much power and a lot of timing involved. Keep the wrist movement to a minimum; it should only bend 45 degrees from start to finish in either direction. Likewise your thumb should only swivel from a parallel to your arm to a 45 degree angle up thru the entire cast - the rest of the stroke comes from the elbow and arm.
The thumb provides a useful guidance device - like the sights of a gun, watching your thumbnail provides a good indication of the tracking and rotation of the rod. I provides a solid brace during the stroke, and is a great technique your casting career should not be without.
Just imagine yourself hammering nails with a FOT grip instead of a TU grip...
Most casters use the thumbs up grip, others a "V" grip, then a smaller percentage use the finger on top grip. There are a lot of in-betweens as well. An easy way to describe the V without pics is that the hand is rotated about the wrist from the thumbs up toward the finger up grip to allow the rod to nestle diagonally across the hand and relieve the thumb a bit.
I believe this adjustment can only provide benefits once the alignment and stroke power of the TU grip has been deeply embedded in the casters technique, so again everything begins with TU.
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