Re: Rod Grip: Thumb or Index Finger?
I almost always use the thumb on top, but occasionally use the index finger when I'm trying to cast very delicately.
I think relating the grip to how one would swing a hammer makes sense. If you're doing general work or really needing to pound, you're probably going to swing it with the thumb on top. If you're putting tacks in and not wanting to mar the surface of the board, you might be more likely to choke up on it and use your index finger.
I think the index finger grip can also sometimes be used as a "trick" to fix a problem in casting. For those who normally cast with the thumb up, if you're having trouble with tailing loops due to overpowering the rod, switching to the index finger grip can often help smooth things out.