It's essentially using a sub-tick size price improvement (but paid to the broker to cover trading fees) to jump the queue at the market. It arguably undermines

It's essentially using a sub-tick size price improvement (but paid to the broker to cover trading fees) to jump the queue at the market. It arguably undermines the existence of price ticks. I think you know this but for the lurkers, the purpose of price ticks is to make the market first-come-first-serve. Without them people can raise the bid by 0.0000000001 to get first in line, making the market last-come-first serve. But as far as shadiness goes, it's a very light shade of gray.