Juro,
Nice graphic illustration however, you have a large arc of the casting hand at the end of the cast, and the rod length appears to become shorter in lenght at the end of the cast.
Lefty's style is for the rod hand to travel on a "shelf" or in a straight line path over the complete casting stroke. Your diagram will not illustrate that motion.
Most casters do have a defined arc to their 1:00 o'clock to 10:00 o'clock stroke but that sort of rotates about the elbow as center of arc (radius) and is a short stroke style.
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"Lack of tension (from acceleration) would let a long aerialized line fall (gravity), causing problems. "
If the single hand caster waits until they feel the line tug on the rod,,,it is too late, the loop unrolls and the line immediately begins falling to the ground. Lefty and Ed teach the cast must begin while the line is not completely unrolled (horizontal candy cane shape) this is one of the reasons for the very slow acceleration,,,,if we acclelerate with too much velocity, the familiar crack will occur with the leader tippet (this is breaking the sound barrier at the instant of reversal.
The initial acceleration must be enough to maintain the line in a level straight line path over the 95% travel until the S_U_A_S,,,if the acceleration is too high the rod will be loaded (bent) and when you come to the final short S_U_A_S there is nothing left, the rod cannot take additional significant loading for the reversal and unloading motion.
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"In spey casting, lack of tension in the beginning of the forward cast can result in the d-loop falling downward or the anchor becoming excessive and killing the cast. This is particularly true with long belly spey lines, so there is a correlation with line length and early acceleration. Extended belly line technique relies upon early acceleration in order to make the casting effort reasonable."
In my initial two handed rod experience, I applied the identical rod path accleration as above (naturally, it is muscle memory, etc.). My casting was o'k for distance and required some fine adjustment for the D-loop formation and follow through to the forward cast. I found it easy to make the D-loop with the quick S_U_A_S in a slight upward arc at the end of the line pick up,,,,I then drop the rod down to the forward cast "shelf" this is only 3"-4" and does not open the D-Loop. The lack of tension at the begining of the forward cast is not a proplem if the D-loop is dynamic or formed with good velocity,,,,,it is ready to begin the forward cast and maintain velocity,,,,if you begin the forward cast as the line is unrolling into the D-loop (right near the end of formation) and not when the loop has slowed down or become almost stationary.
It is all about timing and smooth acceleration,,,,,each rod is different and each line taper length will require adjustment of the accleration & timing. This is something many casters struggle with,,,,they learn one method by rote and cannot fine tune the acceleration when changing rods/lines.
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Rod tip in straight line path??? Try this with a single handed rod, make a back cast with Lefty's long reach to almost horizontal,,,,the S_U_A_S is the short snap at the final end of the rearward arm extension. Now on the forward stroke you can wave the rod tip all over the map,,,,up/down,,,side to side,,,,,and into the very short quick S_U_A_S in the direction you want the line to travel. The line will unroll very nicely toward the target,,,despite the crazy path it follows prior to the S_U_A_S !!! I believe the straight line tip etc. is for eliminating SLACK in the line,,,,,,if you just accelerate to just a stop (and not a short quick S_U_A_S) the straight line tip is more important.
Lefty will make a long cast with his arm almost horizontal on the backcast and finish the forward cast again almost horizontal,,,,,,the rod comes over his shoulder at 90 degrees to the water. Now this is a very large arc and breaks all the conventional rules of casting etc. He also developed a "stab cast" for windy conditions,,,,,the forward cast is completed pointing directly at the target like in fencing,,,the tight small loop and line speed are amazing. I beleive this will work only if you use the technique of S_U_A_S and not just a stop with follow thru.
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I suspect we are on the same page however, you load the rod earlier in the long acceleration stroke and do not use the quick burst of acceleration at the very end. It is probably more comong to most casters than the style of Lefty and Ed Jaworowski.
I have stated several time to both Lefty and Ed that the keystone to their style is the S_U_A_S concept and it is not understood by most fly fishermen. I sometimes just sit and watch other FF cast and rarely see good efficient form, dropping backcasts and punching the rod on the forward cast is the most common.
Regards,
Fred