Tex is pretty much correct. What they do is take the amount that you are renting it for and give you credit for 75% of that amount. That gives you a net rental income (this can be a positive or negative #). That net amount is added or deducted from the household income.
Here is an example:
Mortgage payment: $2000
Rental Income: $2000 (.75)= $1600
Net rental income: $-400
So in theory if your gross monthly income was $10,000 you would only be able to use $9,600 for your debt to income ratio.
The other important thing to remember is that tax and insurance, if not included in the mortgage payment will also count against your net rental income. Hope this helps, let me know if you need more info.