hi,
I am new to servoz and my question is very very basic one. Before i had really seen any system working with servo motors, I had this perception that in a servo motor, the servo remains stationary and only the servo horn and the thing attached to the horn moves. But after watching a lot of things made of servoz, i realized that this functionality too is of servo motors that the horn remains fixed and the servo itself rotates,like shown in picture attached. My question is that when to know…a servo will move and its horn will remain fixed or its horn will move and servo will remain fixed ?
Like in the picture shown…the lowermost servo attached to foot,when provided signal,can attempt to move the servo above it rather than moving the foot ??? cant it ???
I’m not even sure what your question is. And I think your thinking about this too much that it’s confused you.
A servo only has one mechanical function…
It creates a rotational axis that is out putted by a rotating horn.
If the case is fixed then anything attached to the horn will rotate.
If the horn is fixed then anything attached to the case will rotate.
In the image you posted, the end servo is acting as the foot.
Foot fixed to tibia,
Tibia fixed to femur,
Femur fixed to coax.
Whether the horn is fixed or the case, then the end effector attached to the other will rotate.
Yes, u have got my point right…u have replied to exactly what i was asking and yes i am clear about it too now. Thanks 
But to completely convey why i had this question…that i have always worked with Dc gear motors before. In Dc motors its not the matter that if u fix the rototory part, the motor case itself starts moving. In Dc motor case, as u certainly might know, That if u try to fix the rotatory part(stall it) the motor will just burn away. Ofcourse,now i have realized, its not the case with servo motors. Here when u fix the horn(rotatory part), servo will rotate.
Thanks again !
Good I’m glad it helped you. 
Note to add: if you fix the body of a DC gear motor, the shaft will rotate, but if you leave the body free to turn and hold the shaft, the body will rotate. Same principle.
You are right about motors overheating or burning out during prolonged stall. But remember a stall occurs when both case (stator) and shaft (rotor) become fixed simultaneously so that nothing can move. Since electrical energy cannot go to motion, it all goes to heat. DiaLFonZo’s multicopter world is filled with high speed brushless DC motors where the propellors are attached to the cases which spin around shafts fixed to the aircraft frames.