It's a crazy misguided rule.
Scenario: A bets $2 bet, B raises $4 to $6 total, C tosses out two $5 chips (totaling $10, a legal re-raise amount of $4 more).
• if C first says 'call', it's a call
• if C first says 'raise', it's a raise
• if C remains silent, it's a call... wait, wat?
That last one seems nuts to me. C put in a legal raise amount, and it should not matter if it consisted of ten $1 chips, two $5 chips, or a $5 chip and five $1s. It's still a multiple-chip bet totaling a legal raise amount.
The TDA gang got this one wrong, imo.... mostly because there is no need for their rule in the first place.