I'll explain what is perhaps one of the most common strategies.
One first has to understand that it all hinges on the way RCAs are taxed. An RCA is generally a trust. An employer can make a tax-deductible contribution to the RCA.
An RCA is taxed in its own right; what happens is that 50% of the contributions to the RCA are taxed by the CRA, and 50% of the income earned by an RCA is also taxed; then that tax is refunded when payments are made out to the individual. The 50% tax was to get at the time value of money.
What the CRA has found, for example, is that an individual or an employer can make a $100 contribution to an RCA and pay their CRA tax of $50, and then the RCA can make a loan or an investment in essentially a shell corporation controlled by the same individual. Now the money would be outside the RCA and back in the hands of the original contributor.
The trick that occurs is that, since existing rules recognize that investments might lose value, when an investment loses all its value, the RCA can simply apply to the CRA for a refund of the tax, if they turn in all the money they have.
What happens is that this loan is made to a shell company, and that shell company moves the money out. Now the shell company owes money and has no assets, so the value of the RCA's asset is nothing. They get a refund of the RCA tax; they have just taken a circuit out of it.