far from the quaint yet confusing spey cast presentations of yore (and not-so-yore… ) here’s a lot of very good explanations and demonstrations on this foundation cast by one of the best, Eoin Fairgrieve.
the double spey’s main purpose is to easily and safely reposition the line prior to D-loop set up on the downwind side of the caster while creating a pronounced waterborne anchor*.
in the video Eoin points out that he’s on river right ** and has a downstream wind, therefore to cast safely, the D-loop needs to go on the downstream side, something the single spey couldn’t do.
whether using a single or double hand rod, short, middle or long belly line, this cast is not only pleasant to perform but can get you out of tricky situations easily. unfortunately, the whole roll/spey cast family is most often referred to as the casts to use when there’s obstructions behind the caster but that’s just one reason.
often ignored, safety issues such as when fishing from boats with a friend(s), casting very heavy flies or in extreme wind situations are other areas where rolls and spey casts shine.
the only real limitations the double spey might have is casting very far (the single spey definitely outclasses it there) or when using easily water-logged dry flies as they tend to not stay dry when dragged through water during the set up or presentation roll cast but i’ll have an alternative spey cast created just for this purpose for you soon.
on the other hand, the double spey is safer and usually easier to get ‘just right’ than the single spey when using fast-sink tips and specially, big and heavy flies.
to conclude, if you’re new to spey casts or want to get better at them, the best you could do is learn:
a) the roll cast
b) the double spey
c) the single spey
in that order and off both shoulders and preferably with a longish line. (long lines teach us to cast better as there’s less room for mistakes. from there, adapting to shorter lines is a breeze)
once those skills are acquired all other spey casts and spey styles will be a simple matter of slight adjustments as they are only variants of the three above.
*Eoin refers to this as “sustained anchor”, a term coined from the Skagit school. according to Ed Ward, the creator of the term, the sustained anchor, a deliberate and prolonged pause before line reversal into the D-loop can only be applied to Skagi-specific casts with i guess, Skagit lines. this is all being worked on/talked about/debated/and hopefully made clear… these days over on the Sexyloops forum. more on this sustained anchor and other Skagit stuff later.
** river right/right bank- the angler has the current going left to right to the intended fishing area
river left/left bank- the angler has the current going right to left to the intended fishing area
should the angler be in the middle of the river the left or right bank/river designation will be determined as above by current direction and intended fishing area.
if there’s any confusion, imagine you’re on a bridge looking downstream. on the right is right bank, on the left is left bank.