The fingers can have any position and the opposing muscles that move the fingers can still have a large range of equal and opposing forces. That is, the muscles can be anywhere from 'relaxed' with no activation to maximum activation and force as in an isometric exercise. [Isometric means to apply forces but not to move.] It is hard to tell in the pictures whether the muscles are relaxed or supplying forces.
Look at the Nadal picture and hold your hand how he has his. While holding the same pose completely relax all muscles. Then apply maximum forces from opposing muscles without moving as is often done during isometric exercises. You can do both with the same hand position.
I believe for the serve relaxing the muscles allows for a better stretch.
I recently read this so I'm not sure of my interpretation. I used to think of the stretch shortening cycle (SSC) as the extension of the muscle-tendon assembly to near the end of its range of motion - like stretching that muscle-tendon assembly like a rubber band. Then, if the prestretched muscle-tendon is promptly allowed to shorten during the athletic movement, it adds force. Using prestretched muscles may be naturally faster also.
However, I now believe from my interpretation of some recent research that it is not the muscle-tendon assembly that stretches - it is mostly a large protein molecule called Titin in each muscle cell (sacomere). The interesting thing is that Titin can probably chose to stretch well before the muscle-tendon assembly is near the end of its range of motion - maybe at any muscle length. ??
See Figure 7 for the proposed interaction of Titin in each muscle cell.
Regulation of muscle force in the absence of actin-myosin-based cross-bridge interaction
See reply #5
Biomechanical References for Tennis Strokes
Is an EMG signal present when the muscle shortens using only pre-stretch?
Maybe if Nadal has applied a lot of muscle force to hold his hand as in the picture then the stretching characteristics of his muscles would be much different. For example, maybe his arm acts like a rigid mass, a counterweight? If relaxed it would not act the same. ? Federer's off hand looks relaxed. ?