The Perry Poke can be used as a get out of jail free cast when you have found it necessary to abort your Spey cast prematurely usually because your anchor point did not achieve the required position, thus still allowing you to make a very efficient cast.
It can also be adapted to be used in several situations from recovering after a badly set up Spey cast to delivering a very heavy or bulky fly to it's 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 wherever you have placed your fly in the early stages of the set up of your cast, you can then dump (poke the rod tip) down on the water in front of you to create some slack line which you can then transfer to 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 slack line you have dumped on the water in front of you will afford you maximum casting weight when placed into your D loop prior to your delivery.