Pole vector contraints are useful for elbows and knees.
This is how I use them. First create your joint chain, in the example shown I have created a simple leg. Then create an ikRPsolver from the hip to the ankle. Next I create a locator in front of the knee. Now select the locator and the IK handle and create a Pole Vector contraint. Now click on the locator and move it in the X-axis, hey presto a knee control!
Do the same thing for the arms and you have elbow controls.
If you look closely at the channel box you will see that Pole Vector is highlighted in blue denoting the Pole Vector constraint.
Hope this helps.
Tickety boo and ta ta for a bit...