What is FlexMotion? 
FlexMotion is an Effect that controls hierarchies of objects using points on a Curve tool to adjust and animate them.  It is similar to Flex, except that it controls multiple objects or items.  The chain of objects conform their shape to the shape of the Curve.  For example, you'd have a bone setup and apply FlexMotion, then animate the Curve instead of the bones themselves.  Some people are already calling this "spline IK".

And if you use FlexMotion (Curve) with a set of bones, the bones can stretch and flex like muscles if you want!  In fact, just about anything can be part of a hierarchy that will be manipulated with FlexMotion -- bones, null objects, polygonal objects, etc.

Why do I want it?
Because it's damn cool! This effect is ideal for tails and tentacles. It is also perfect for "snaking" and object along a path. Other not so obvious uses are using the curve to animate lips and eyebrows. 

How do I set up FlexMotion?
          1) Create a hierarchy of whatever you want--bones, objects, etc.

2) Select the first bone (or object) in the hierarchy, then go to the Setup tab and select FlexMotion from the pulldown list on the Add block.  Click on the button to add it.

3) On the Curve block, turn on Point Add and add control points by clicking where you want them in the World View.  When you're finished, exit the setup mode by clicking to Animate or one of the other main tabs. 

Then just select a point and move it.  The bones should go with it.  

If you want to change the FlexMotion parameters, select Curve in the Item List and then go to the Curve block that will appear.  Or select FlexMotion in the Item List and go to the MotionBlend block that will appear.

Setup mode:
Setup mode is where you create the FlexMotion Curve and perform all of the setup.  What you are doing here is creating the Curve points and adjusting their "rest position."  In other words, the position they're in without any animation--their default position.

How can I use FlexMotion to make muscles?
Muscles can be made with the Squash & Stretch feature of FlexMotion.  Just set up the bones and apply a FlexMotion curve, then set the Match Tool  (MotionBend block) to  Absolute Length or Uniform Curve Segments, turn on Scale Children Xform, and set the Elasticity to Squash & Stretch.  This type of muscle is especially useful for things like mouths and eyebrows.

Additional Information:
There are four blocks in Setup that associated with FlexMotion.  They are:  Curve, Curve Options, MotionBend, and Point.  And be sure to do the FlexMotion tutorial.