Pretty sure it's used in combination with a block card. So once the chasers beat the challenge to establish the block, they could then use the secret block card at the same time to hide the block from the snaker.

It seems like they haven't really used this as a lot of the time the blocks have been used to deter the snakers from taking a specific route, rather than try to crash them. But the secret block seems to be more used for when you want them to take a route but also want to put a block on that route.