In a subject to situation title to the property is transferred to the buyer, but the original loan stays in the original borrower's name. The buyer makes the monthly payments, but the loan does not show up on their credit report thus not impacting their ability to get more credit in the future.
So for instance, if you purchased 7 rental properties over the next year with your own credit, you might reach a point where you could not get any additional mortgages. Now, let's say that you wanted to buy a new personal residence, your ratios might be weak and your loan turned down. If however, all those properties had been purchased subject to the loans, none of them would show up on your credit and your ratios would not be affected, so you would be able to get your personal loan.
You are still responsible for making payments on all of the loans. It is not about getting out of paying - it jst doesn't affect your credit. In other words, you still need to make sure that your portfolio is performing well enough to take on responsibility for an additional loan before purchasing a house subject to.