Pretty simple, really.
Forgiveness means if you miss the ding it is likely to start closer to your intended aim line.
Given that WGT have never really been able to explain what balance is, just that it is a linking factor between precision and forgiveness, a theory is this:
Precision is not only how closely to your aim a dinged putt goes, but is also the degree of predictability that a missed ding goes a given amount off line.
In a way, this makes precision an opposition to forgiveness, because along with high forgiveness increasing the chance of it going more towards your original aim line with a miss, high precision is making it go off line in proportion to how much you miss the ding.
So I think balance is some linking factor in the equations that decreases the precision's opposition to forgiveness, ie. the higher the balance the more precision AND forgiveness you can have in tandem.
Got it?