The logic being that nearly every system one has seen , based on form, uses that percentage range in the rules somewhere, when it comes to Handicapping a race.

Thus, they tend to be over bet by the general public.
Thus their market price is less than their true price ability to win the race.

Remember the art in laying is taking on horses priced below their true value chance of winning.
Example
Live market price 4.50
Accessed price chance 5.00 (20% chance of winning)

Therefore if I can successfully keep doing this for this price range for example.
over 100 races all exactly 4.50 & only win 20% of their races.
4.50 x 20 = Payout 90.00
Outlay 100
Prof +10
10% POT

Also 2nd Favs are more vulnerable to falling over than say the 1st favs
2nd favs general SR is 20%
This gives us a strong base to start with.

That's the general idea of why it works.

Like always with these things .
Its imperative that you do your own due diligence checks before betting any real money on any system one wishes to pursue, over approx 150 bets.
Or over another data base, using unseen data.
Then go from there.

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Cheers.