Hi Paul
Exactly the same as "the caster has started the forward cast before the line has fully straightened".
Visualising D - nope - have a look at the clip of Steve Rajeff - his loops are thrown back and during each pause while the loop is unrolling as it goes back, his preparation for the forward stroke is to drop the rod tip by rotating the rod backwards. He opens the arc rather than trying to punch forward through the same arc he used for the backcast - he's well aware that on the backcast he was simply carrying line - whereas on that final drive he wants to shoot a load of line - that requires more acceleration and will bend the rod more deeply - so to maintain a relatively linear tip path he needs to use a wider casting arc.
There is no need for that drifting motion to be sudden - bearing in mind it needs to be timed - the line hand plays a part in feeling tension in the line and feeding back from the haul, I think I use the line hand feeding line up as a cue for my rod drifting back???
No that doesn't come into the concave path - it happens between strokes during the pause. Seriously - try it - during the pause as you feed line up the rod and bring your hauling hand back to position let your rod drift back a few degrees. Thing is Paul I would not be surprised if you don't already do it unconsciously and very surprised if you can't consciously do it.
Since we were with SANA and SCAIC and all that I've been exposed to a load of other stuff, lots coming from Sexyloops and FFF instructors and some UK guys. The variable casting arc was the key concept - creep and drift followed. As far as I know the term creep was coined by Bill Gammel and he explained it at length on SL - Bill is a highly respected US casting instructor and with his father co-authored The Essentials of Fly Casting. The first person to show me how to drift was either Hywell Morgan or Jim Tomlinson - its been around the Tournament casting circuit for many many years.
Bill Gammel and his father worked out what they think are the underlying essentials for any cast:
1. The rod tip must travel along a straight line.
2. Vary the size of the casting arc according to the amount of line outside the rod tip.
3. The pause between each cast to allow the line to straighten must get longer as the line gets longer (correct timing). Learn to vary the timing and the stroke length to maintain the straight-line path of the rod tip.
4. The power must be applied at the proper place, at the proper time.
5. Slack must be kept to a minimum.
Those are based on observation and casting mechanics and I can't pick any significant holes in them (- see below.) As far as I know those essentials guide the mechanics used when FFF instructors teach and as I read the AAPGAI syllabus it looks like they have adopted them too although not explicitly. The great thing about those is that they seem to apply to any and all casting styles - teach with those in mind and you need not teach a casting style.
Lefty is talking absolute bollocks about the hand path if he thinks that applies to all casts or all casting styles - try this
http://www.sexyloops.com/movies/cnlpaul/cnlpaul2.mpg or trace the hand paths in those Rajeff clips.
The style Lefty is advocating is essentially a sidecast he's saying the plane of the cast should be aligned front and back. The guys he's humiliating doesn't use his style. There are simply so many examples of where that idea of sliding the elbow along a bookshelf is simply wrong advice - try it with an upright overhead style. Paul Arden's old Flip Flop style is an extreme example but it works - he's since changed style because Flip Flop seemed to max out at around 115 to 120ft. Even with that style the hand path changes as the carry changes - becomes steeper and shorter with a short line - longer and more horizontal with a long line. What he can't do with that is gain loft. Look at a shooting head tournament caster and because they are using a relatively short head they can angle the backcast down and track the rod so it ends high - angling the casting plane up.
Look at us when we teach novices to deal with wind - the casting arc is rotated so the line path is angled up or down - into a headwind we throw a high backcast and a low forward cast. How exactly can we do that without changing the hand path - how on earth can Lefty do that and maintain that flat hand path - he can't!
This is a far better piece by Lefty - but look at the first few pickup and laydown casts - rod side foot forwards and note his handpath. Then the principles - eliminate slack - absolutely right! If I have any problems with Lefty its that he says we should teach casting without being dogmatic about style then when he teaches he is dogmatic about casting style.
Lefty's principles are here
http://www.hoaff.org/newsletters/NL2003-04.pdf if your interested. I have a few issues with Lefty's Principles in this form - bear in mind these principles are meant to apply to ALL casts.
1 - Getting the end of the line moving - aka eliminating slack - sounds fine as it stands but excludes all roll and Spey casts.
2 - Accelerating the hand sounds fine but if you try accelerating your rod hand without rotating the rod you will get no speed gain whatsoever from the rod. Moving your arm so the rod angle changes and the rod accelerates and loads as it rotates is what he means.
The sudden stop was sometimes called the Power Snap it was never called the power stoke - it was called the stop. (There's good reason to think a sudden stop is not essential -
http://www.sexyloops.com/articles/rodcast.shtml)
The size of the loop is NOT determined solely by by the distance the rod moves in the final part of the speed up and stop. A wide loop is often a product of the whole casting motion - the rod is never properly loaded and we throw a big wide loop - what Mel Krieger calls a non-loop.
3 - The line does not always go in the direction the rod speeds up and stops. In a snap cast the rod accelerates to a stop going in the opposite direction to the loop. In any curve and many other presentation casts the rod and line are not producing the simple motions Lefty clearly has in mind. Remember according to LK these apply to ALL casts.
4 - I sort of agree with him on this one - long cast long stroke - short cast short stroke. When carrying line, false casting, I would argue that the stroke length is proportional to the length of line in the air.
However, again there are plenty examples where that does not apply - check a caster using a shooting head style (see the clip of Steve Rajeff) - their stokes always tend to be short and clipped and their arms rarely extend far behind their bodies. Most DH casters already know they can't extend their arm behind to lengthen their stroke. Underhand style with a DH rod uses a very short clipped stoke....more if required.
If you want me to go on about Lefty I can - he also says that casting instructors commonly teach that the stroke using a clock face - very very few now use that analogy. He argues that the Haul should only be about 6 inches long - he's wrong - it can be as long as your arm, it's easier to make a long fast haul if you don't stand the way he stands....probably the only certainty about the haul is that the haul should not end before the rod comes straight and the loop forms.
Magnus