Leverage

The fly-rod can be used as a lever but perhaps not in the same way as we visualise a normal lever, such as in the case crowbar which can be used for applying a lot of force with very little physical effort.
When we want to we can use the fly-rod to the move the fly-line a great distance with only a very small movement of the hand (or hands in the case of a double-handed fly-rod), we can use the leverage provided by the fly-rod. This helps us to generate fast rod-tip speeds when we wish to have energy in the cast or to create wide sweeping movements when we need to move long lengths of fly-line in a controlled manner.
The amount of movement of the rod-tip relative to the casing hand(s) depends on the hinging point or fulcrum. The closer the fulcrum is to where the force is applied, the greater the leverage.
A longer fly rod will provide greater leverage but it can also put a greater strain on the casting hand(s). It is also possible to increase leverage by casting using the hinging of the elbow joint, rather than rotating the wrist. Because the elbow is a stronger joint it can also apply more power. Alternatively, in the case of a double-handed fly-rod the hands can be positioned closer together and hence a faster acceleration rate and higher rod-tip velocity can be generated, which is ideal for casting shorter shooting-heads.
Leverage can also be used for magnifying a movement as in the case of forming a D-shaped or V-shaped loop. Instead of moving the hand(s) it is better to move the rod-butt, by pivoting it in a manner that mimics the shape of the path that the rod-tip is required to follow.
By doing this it is possible to achieve wide sweeping movements of the rod-tip with small movements of the hand(s).
Examples of when such sweeping movements are used are forming the D loop, peeling off an anchor and lifting the fly line.