I would direct to a search but this one might be hard to isolate with keywords.
"Spey" is a casting style ideally suited for river fishing (salmon, steelhead, and other species) on the 'swing'. It solves some very important problems for the river angler:
1) requires very little room behind the angler on steep or brushy banks
2) allows casting to fishing distance with change of direction in as little as one back and forth casting motion
3) promotes methods for dealing properly with wind on either bank
4) increases the amount of time per outing that the fly is in the water
5) improves line control (mending, line tension)
6) relieves fatigue once practiced
How is this cast made?
Instead of extending the line fully behind the angler as in a common fly cast, the backcast is tucked backward beneath the tip of the rod and kept folded in the air such that the leader and end of the line come to light on the waters surface for an instant
Freeze that: at that instant, the line has formed the shape of a D (or vee) to the rear of the angler, and the bottom half (anchor) is 'kissing' the water. The rod is held in a similar position to that of an overhand cast at the end of the backcast, the proverbial 2pm position. This is referred to as the "D-loop".
Go: with the momentum of the loop going backward and the touch of the line on the water, a normal forward cast creates enough tension to load the rod and with a smooth acceleration and hard stop the line flies forward much like, if not exactly like, an overhand cast.
Timing is key, making the forward cast too early kicks out the anchor; making it too late lets the line drop to create too much tension on the surface. Slurp, sizzle, plop.
When the fly has swung to the shoreline, the caster can lift the line, sweep the D-loop alongside opposite the target out in the river, and let the cast fly. Only about half of the length of the head portion was needed behind him/her. When combined with shooting of line, the distance can multiply the length of your backcast 3 or 4 times.
But a picture paints a thousand words, go check out http://www.speypages.com
and watch a few of the videos. Speyclave.com is one and the same with the Forum so asking a question like this will get you lots of answers on the Spey board.
Hope that helps!