I think the "thumb on top" grip is the most anatomically uncomfortable grip to use for the backcast (except for the forefinger on top that is)
and results in poor backcast loops with nearly every beginer and most others as well - unless they are only casting short distances.
The thumb on top is a powerful grip for the forward cast though.
But there are no rules against using a palm forward, "V" grip, or three point grip for the backcast, then simply rotating the forearm 90 degrees (or whatever) to get the thumb on top. Or sliding the thumb over onto the top for the forward cast. To do the former means that the reel will be parallel to the ground on the backcast and perpendicular to the ground on the forward cast.
If doing constant blind casting, as in working shorelines for bass or dredging deep in lakes for trout, or casting to the mangroves for snook and holed up redfish, I change my grip on the forward cast throughout the day. The muscles used in a thumb on top are different from the ones used in a palm forward grip and switching muscle groups helps with fatigue.
I've only tried the forefinger on top recently, and such a grip would never have occurred to me if I hadn't read about it. It's main use to me would be to get newcomers to the sport into the habit of not driving the backcast into the ground or water behind them.
I also use the middle finger to trap the line when stripping for several reasons. One is that there is a finger on either side of it so when you clamp down on the line, the line has to make two 90 degree turns to get away, instead of only one when using the index finger.
But the other is that is is the least important digit on the rod handle in terms of handling the rod. The index finger and thumb and the two fingers fartherst away from them are what gives you the leverage and control over the rod - both during the cast (especially during the "power snap" and stop for the little fingers), and while mending , setting the hook and so on.
And the other thing, for you guys up north who dredge for big brookies the day after ice out, the middle finger stayes warmer than the index finger while ice is buillding up in your guides.
I don't have to deal with that anymore - but I've seen some of your pictures, and it looks like "stripping with the rod underwater" weather to me.
Really, though, like grip, where you hold the line is whatever works best for you.
---------- Post added at 12:00 AM ---------- Previous post was Yesterday at 10:45 PM ----------
Assuming the above is the correct way (also assuming of course, at that time there is no need to double- or single-haul, or for a false cast), when is the correct time to release the line from under that finger?
At the abrupt stop on the forward cast? Just after the stop as the rod is being lowered? Wait 'till the rod is level and line is close to completing turning over?
That question deserves a complete thread and IS the million dollar question. In fact, that question started a debate 3 weeks ago at a competitive distance casting website comprised of the world's best distance casters, and there have so far been 216 replies to that basic question. The answer is that nobody really knows for sure.
Just two days ago, I received two high quallity microswitches I ordered which I will hook up to a battery and a trailer tail-light. I will tape the switch between two fingers of my line hand and hold the line between my thumb and the switch. When I release the line, the tail light will come on.
I may need to make some adjustments to the switch button, but once done, I will send the two setups to a couple guys in Europe who are always at the top of the international competitions and who have high speed video cameras. My video camera only does NTS (30 frames/second) which is useless for this since my rod moves almost completely through its rotation from one frame to the next.
Besides, I want to see exactly when a professional distance champion releases on a great cast.
I suspect that single-handed casts are not released until right before maximum counterflex after the stop and after the rod has passed the "rod straight position". Several guys claim that they release before "rod straight position" and never stop (on the final forward cast).
So if the best distance casters in the world don't even know when they release, I wouldn't worry too much about it for a while if I were you.
For myself, I stop when I run out of arm on long casts which is when I finish up the haul. And I think I release shortly after the rod straightens out and has already started counterflexing the opposite way from the bend when it's loaded. But lilke them, I don't know for sure.
I will keep you informed of progress if you are interested.