Here are three good instructional videos I found in YouTube showing a casting stroke that is very fundamental to all Spey cast. These three videos should be viewed in succession in order to appreciate the principles behind the stroke.
When I started helping friends with their Spey casting, I noticed that most of them encountered problem performing this transitional stroke to the forward cast. I call this transition stroke the upward spiral (rod tip path) whereas it is referred to as climbing curve in this video. People sometimes also talk about circle-up / rounding up to form the D-loop.
Whatever name you call it, it is undeniably the most important move to master in Spey casting. Many who pick up Spey casting find this move difficult to master initially. Some of the most common problem encountered by new Speyers are:
1) Unable to round-up or spiral-up smoothly
2) Dipping the rod tip too low behind while performing the round-up
3) Paused too long behind before powering forward
FP does a great job here with these three instructional videos emphasizing on the top hand being the pivot, powering with the underhand and how to correctly angle your rod to prevent rod tip dipping behind. Learn this move well and you will be well on your way to better Spey casting.