On the other hand, the automatic clock-sense circuitry can monitor primary and secondary clock sources and automatically switch from the primary clock to the secondary clock in the event that the primary clock stops toggling.
The manual clock switchover feature using EXTSWITCH is useful when clock sources originate from multiple cards on the backplane, requiring a manual switchover between two frequencies of operation. You should choose the secondary clock frequency so that the voltage controlled oscillator (VCO) operates within the recommended range of 500 to 1100 MHz. You can set the M and N counters accordingly to keep the VCO operating frequency in the recommended range.
You can use the EXTSWITCH feature for applications where the user wants to switch back to the primary clock from the secondary, once the primary is stable again.