The price of real estate reacts to both demand for property and the rate of inflation and rate of income growth. Mortgage rates generally move as treasury rates move. See this paragraph:
As we mentioned, intermediate term bonds and long-term mortgages (more properly, Mortgage-Backed Securities, or MBS) compete for the same fixed-income investor dollar. Treasury issues are 100% guaranteed to be repaid, but mortgages are not; therefore mortgages carry more risk of default or early repayment, which could potentially disturb the return on the investment. Therefore, mortgage rates must be priced higher to compensate for that risk.
But how much higher are mortgages priced? In a normal market, the average "spread" or markup above the 100% secured Treasury is about 170 basis points, or 1.7%. That markup -- the spread relationship -- widens and contracts with a range of market conditions, investor appetites and supply of available product -- as well as the presence of competing investment opportunities, like corporate bonds or domestic (or foreign) equity markets
Source: What Moves Mortgage Rates?
And when the stock market crashes, investors tend to run to bonds and treasuries, which causes prices to go up and treasury yields to drop. Theoretically, this would also cause mortgage rates to drop, although most mortgage rates have a base price below which they cannot fall.
How easy is it to profit from recent stock market drops and at what frequency?
Incredibly difficult. The issue with your strategy is that you cannot predict the bottom of the market (at least us mortals can't). Just take the month of August for example. Stocks fell something like 15%? After the first 5-10% drop, people felt that the bottom was there, so they rushed in, only to have the market fall even more.
How will you know when to invest? Even if the market falls by 50%, and there's a huge buying opportunity, and you increase the mortgage on your house, odds are your rates will increase because of the equity you take out. What if the market stays low for a very long time? Will you be able to maintain mortgage payments? Japan's stock bubble popped in the early 90's, and they've had two lost decade's now.
Furthermore, there are issues of liquidity. What if you need more capital? Can you just sell a property or can you buy now property to draw equity against? What if the market is moving too fast for you to take advantage of. Don't ignore transaction costs and taxes either.
Overall, there are a lot of ways that your idea can go wrong, and not many ways it can go right.