The Perry Poke can be used as a get out of jail free cast when you have found it necessary to abort a Spey cast prematurely, usually because your anchor point did not achieve the required position, thus still allowing you to make a very efficient cast.
Apart from being used to recover a badly set up Spey cast It can also be adapted to several other situations, from assisting with the delivery of a very heavy or bulky fly, to being a very useful cast when using Skagit lines as it will easily dig out very fast sinking tips and allow you to deliver them very efficiently to your target.
As long as your D loop is eventually formed directly behind the fly and preferably in the direction you are going to deliver to (as with all Spey casts) then the cast will be efficient (if delivered correctly).
So basically, wherever you have placed your fly in the early stages of the set up of your cast, you can use this technique to dump line (poke the rod tip) down on the water in front of you, affording you time to transfer this line from the water into your D loop under tension. This newly formed D loop can then be delivered out across the water to great effect.
If you have purposely created your anchor point in the correct position on the water when executing this cast then you can be sure that the line you have dumped on the water in front of you will then afford you maximum casting weight when placed into your D loop prior to your delivery.