Here's how they did it. Rent was $550/month with a one year lease, which works out to $6600/year.
When the least expired, you had the option of going month to month, or signing another lease. Month to month would be $600/month ($7200/year). However, they said, if you'll sign another one year lease, they'll let you keep the old rate ($6600/year), which will be implemented by giving you one month free. That is, you'll pay $0 for January, then $600/month for the remaining 11 months of the year, bringing the total to $6600/year.
A year later, when it was time to renew again, they told people rent would probably be going up soon, but if they renewed now for another year, they could avoid the increase and just keep paying $600/month.
Since that is what people were already paying, most did not see this as a rent increase. Yet they would be paying $7200 for the year, as opposed to $6600 for the year before--a $600 increase--because this time there was no free month for signing the lease.
It also sounds vaguely similar to the thing you hear factory workers do.
Bob is paid weekly, on Thursday. Bob starts by being frugal. This means that instead of having to use his wages on Thursday he can push it back a day to Friday. He still gets paid on Thursday.
He keeps doing this - keeping 8 days between each time he takes his money out of the bank.
After 7 weeks Bob gets a double pay packet.
I know a few people who tried this, and I know a couple of people who managed to last for the 7 / 8 weeks needed. Most of them got more benefit from frugal spending and a bit of planning than from the double pay at the end, but that's a nice treat too.
It's just a standard 1-month free signup discount followed by annual increases.
There's no reason the discount has to be for the full rental amount in a month. To obscure an increase from $550/month to $575/month, the offer could be be $300 off for January: 12 x $550 = $275 + 11 x $575.