This command will insert a new joint under the given or selected joint. If the given joint has child joints, they will be reparented under the new inserted joint. The given joint(or selected joint) should not have skin attached. The command works on the selected joint. No options or flags are necessary.
Derived from mel command maya.cmds.insertJoint
Example:
import pymel.core as pm
import maya.cmds as cmds
# Will insert a new joint under joint2. Child joints of joint2 will be
# under the new inserted joint.
pm.insertJoint( 'joint2' )