Quoted from taylor34:

See this example:
You buy a house for $90k. You put in $18k as a 20% downpayment. You then put $30k in reno into it, so you've spent $48k. If you go to the bank and refi, they will go do an appraisal. Different banks have different rules, but a lot of them will allow you to take up to 80% of the appraised value in a loan. So if the house now appraises at $160k, you can take up to $128k out as a loan, paying off the original loan + your $18k down + your reno budget.
It's no different than refinancing your personal residence...if you have a $300k house and you owe $150k on it, they'll let you take out a loan up to $240k on it if it appraises that high.
His post on the strategy is correct. The downsides are that if the housing market ever goes down more than 20% you'll be in the red, and it takes a lot of work to renovate a house (and find deals). It's kind of like pinball buying...if you buy on pinside (like the MLS) you'll rarely find a deal. But if you find the right home owner of a pinball machine (or house) you can get it much cheaper than normal.