The tiebreaker occurs in this revival of Chain Reaction after the betting round when the dollar amounts are tied. The two teams have to solve separate speed chains, very similar to the speed chains between each round in the 2006-07 version of Chain Reaction, meaning it is a four-word chain. Each team gets seven seconds to contemplate what the two words between the two clues (at top and bottom) and then have to answer immediately to Catherwood.
The team who scored most recently (who caused the tie in the last play of the betting round) gets the choice between "Chain A" and "Chain B". Whichever team gets their speed chain correct first, wins. This is similar to the "Sudden Death" rule on Family Feud. For example, if the first team (who caused the tie in the last play of the betting round) gets their chain correct, they advance to the bonus round. The other team does not even have a shot at it and is automatically eliminated.
This Chain Reaction tiebreaker is an excellent tiebreaker and much better than the Idiotest one, which does not count on smarts or a tiebreaker puzzle but rather "who solved first by a tenth of a second" rule.