Question about William Nickerson's way

[Willis Seng]

Can anyone clarify William Nickerson's steps in real estate investing to me. He uses a combination of flipping properties and renting them out, but i dont understand why you would need to rent them out. His method is to purchase a beat-up property in need of rehab, renovate and rehab, rent it for 2 years, and then sell it for 25% profit. How would you renovate with your property occupied with tenants? Wouldn't be better to sell the house right away so you wouldnt have to get caught up in morgage payments? I dunno, maybe it explains later in the book, I havent finished it yet, but i cant wait! i must know! thanks!