Free Money
Take the best over anyone is posting and the best under anyone is posting. If those two prices together imply less than a hundred percent, the number pays whatever happens.
How this is found
Every price implies a probability. On a healthy two-sided market the over and the under add up to something above 100% — that surplus is the book's fee. But the best over and the best under do not have to come from the same book, and when two books disagree far enough, the two best prices can add up to less than 100%. That is an arbitrage, and it is arithmetic rather than opinion.
Why the board is usually empty
It should be. Prop markets are sharper than they were and these windows close in minutes. An empty board is the honest answer and you will see it most days. What you should not see is a board inventing opportunities to look busy.
Before you act on one
Check that both numbers are the same — an over on 4.5 and an under on 5.5 is a middle, not an arb, and it carries real risk. Check the prices are still live; stale lines are the single most common false positive. And know that books limit accounts that do this often.
The everyday version of this idea is the gap, which does not need two books to disagree, only one to be generous. Shop your number on the ladder first.