A Question About The 50% Rule

[Chris StPeter]

Hello, I'm new to real estate investing, and have a question about the 50% rule that was discussed in the new to real estate course.  I understand how it works, you take the monthly rent, divide it in half, subtract the mortgage payment, and what's left is your cash flow. My question is this: why do I subtract 50% from the monthly rent before I subtract the mortgage? where does that 50% go? am I missing something?

Thanks,

Adam