The index report available means that the Z marker pulse has been passed or crossed. The Z marker pulse occurs once per rev of the motor, but it is a very short signal. It is used for homing with in one revolution. The trick is... you can pass the Z marker... but not be able to stop on it.
Example: I power up the motor and start a velocity move. Since we use an incremental encoder.. we don't know where the Z marker pulse is. When the Z marker is passed... we trap an encoder position in the "index register". This allows us to move back to that encoder position so that we are aligned directly on the Z marker pulse.
If you power up the motor and move the shaft... the Index register will trap a number in encoder counts when the Z marker pulse is detected. This number could be anything... but for example... lets say it trapped 300 in the index register. If the motor has 2000 pulses per rev... the next time a revolution occurs... the index register would have a value of 2300. ...the next time a revolution occurs... the index register would have a value of 4300.