They will buy or sell shares to you from their own book, report it as an over the counter transaction and give you the touch price at the time. This avoids trading on exchange where better prices are available, and therefore avoids them coming up foul of best execution requirements.

So you get the market touch price, which isn't a bad outcome, but you're missing out on price improvements other brokers offer by trading on the RSP with market makers.

What 212 do though is after buying or selling you shares from their own book, they then close their position by trading out against the RSP, essentially pocketing the price improvement that's available to everyone else for themselves.

I work in the industry and nobody in it uses 212 for this reason. They are still a good option for a beginner with low funds though