For anyone who doesn't know, an arb or a scalp is when there is an overlay between two books allowing you to back both sides and guarantee a profit.
With smaller markets, and especially props, you will often see huge variations in numbers from one book to another. It's not at all uncommon to see:
Book A: Manning U2.5 TD -140/+110
Book B: Manning O2.5TD -130/+100
One option you have is betting $100 at one place and $105 at the other and guaranteeing yourself $5. That is called an even scalp.
But if you like one side more than the other, or you know which number is the off market number, you can make an uneven scalp and essentially manipulate the number into something more favorable.
Let's say you think that Manning O2.5 TD should be even money, so the book that is offering O2.5 +110 has value.
If you intend to bet $100 to win $110 on O2.5, you can instead bet $200 to win $220 on O2.5 at book A, and $100 on U2.5 +100 at book B
If Manning O2.5 wins, you win 220 from book A, and lose 100 for a profit of $120.
If it loses you lose 200 from Book A and lose 100 from Book B for a loss of 100.
You have turned a +110 bet into a +120 bet.
The more you are able to get down on each side, the more the line will improve. If you were able to get $500 at Book A and $400 at Book B, your bet would now become +150.
There are two major issues with doing this. One is if you are playing offshore, it's such a pain in the ass moving money around, it might not be worth the effort. The second is if you are betting large amount on props and winning, you are going to lose an out. If you are already betting $200 on props, this will likely become a problem. But if you're only betting $25-$50, this is definitely a strategy you can use to help yourself get some better numbers.