Two gaps pointing opposite ways, overlapping each other. Where they overlap, both sides of the imbalance have already been paid for — and that overlap is the zone price respects.
Why it is called balanced — A single gap is one-sided: only buyers, or only sellers, traded through it. Two opposing gaps over the same prices means both sides have been filled there — the price is balanced, not skewed.
What you mark — Not the two gaps. The overlap, and only the overlap. Outside it you are back to reading one ordinary imbalance.
Why it holds — Price has no unfinished business inside a balanced zone, so it tends to be rejected from it rather than delivered through it.