Basically, the idea is that, in an economy with highly developed mortgage markets and a proliferation of mark-to-market accounting rules, changes in the value of collateral and wealth have a greater impact on the economy.
In such an environment, borrowers’ balance sheets tend to fluctuate more, which leads to a higher volatility in the amount of funds banks are willing to lend to households and firms. Obviously, greater changes in the supply of credit will translate into greater changes in consumption and investment, thereby strengthening monetary policy transmission.