Back around 2003, I stumbled on a $30,000 maiden claiming race at Oaklawn Park that was turning out to be strong. It was a field of 12 and the winner, a first timer, had won by ten-lengths. He came back to win a starter allowance for fun and then another allowance race. At this point, 5 of the first 6 runners who had run back, won their next start. It was a key race to be sure. What made it even stronger was those winning next out had run out of the money in the key race which meant the general public wasn't privy to the strength of the race. I made a nice profit on a couple of others who won from that race later that spring.
Steve Davidowitz's book, Betting Thoroughbreds, amplified the importance of key races. I'd say it's the angle I weight on top of all others. I'm a class player and nothing speaks to class more than the company you keep.
Not all "key" races are the same. Davidowitz taught us to focus on horses who came back to win out of a certain race. Back in the 1980's, I used to copy all the result charts for my home track in Minnesota and highlight the winners coming back. It wasn't long after that I started to consider horses who ran better than expected next out, or moved up in class and outran their odds. Sometimes a second place finish at a much higher level is more important than a win at the same level.
Take the February 16th, 2019 maiden race at Gulfstream Park won by Shancelot. We should have taken notice of this race the minute Bodexpress ran 2nd at 72-1 in the Grade 1 Florida Derby as a maiden next time out. It was a legitimate effort that has since been validated by his current form. Those who were paying attention, and were not tipped off by the italicized winners text from Bodexpress, got a gift on Bandon Woods when he came back to crush at 5-2 in a MSW at KEE a week later.
Notice the Beyer Figures on the comebackers, as well. They were all very high which indicated this was a strong maiden race. The key is identifying a strong race before it is plain for all to see. Obviously, Shancelot and Fort Worth were strong plays coming back out of this race but the 1-5 odds on both made it void of value.
Finding a negative key race is equally important. When well-meant runners flop next out at the same level (or even below), it's worth taking note. Be careful here. Horses run poorly for a variety of reasons (wrong surface, bad trip, not healthy, etc.). Make sure there is a preponderance of evidence before labeling a race weak.
Just like anything else in factoring information in the handicapping process, there isn't a Holy Grail, and there often isn't a black and white answer. When Bodexpress ran 2nd in the Florida Derby, some might have said that he hung around on a slow pace when the winner and runner-up ran one-two in a merry-go-round. I know I felt that way. Those who felt that Bodexpress was a deserving G1 placed colt could now inflate their opinion on the others he ran with in the maiden race listed above.
Formulator will help you identify how well horses ran back out of a race. Don't assume just because an also-ran won their next start that it bolsters the strength of the race he is exiting. Often times that horse was compromised in the previous race (off-track, bad break, etc.).
Finally, key races, just like Beyer Figures, had more value before they were italicized and bolded in the Daily Racing Form. Years ago, those that tracked this information themselves had an edge. Now the Racing Form helps those with the way the PP's are printed. Key race are still an important angle and should be used in conjunction with other relevant information.