Checking the box enables ground balance and from what I understand you should not need it 95% of the time. I personally have not used it in the 3 years I've had the CTX. I've have not hunted in really difficult soil conditions or in salt water. If your auto sensitivity is really low, below 8, then I think you would want to check the box and use the coil pumping method for ground balancing. Once that is done the CTX will then automatically keep it ground balanced. When it is not checked I think the CTX is not ground balancing at all.
This Is in the manual:
"Ground Balance should not be enabled in most detecting conditions where ground
mineralization is mild. This includes most beaches, parks and sports grounds."
Perform Auto Ground Balance
Ground Balance must be enabled to perform Auto Ground Balance.