The idea is that taking a secured loan out using an asset as collateral would be a taxable event for that asset.
That is to say, if you buy a house for $400,000 and it appreciates to be worth $850,000 then take a home equity loan out against the house, you would owe capital gains on the $450,000 appreciation.
With the current $250,000 capital gains exclusion for primary residence, this would result in ~$30,000 of capital gains tax.