To see what I mean, let’s imagine a PC party trekking through the jungle. Nearby are some bad guys, waiting in ambush. It would be interesting to determine if the PCs notice the ambush, so that calls for a roll. The GM has set the Action Cost for a “Realize” type action at 18 - fairly steep as things go. Let’s imagine two different PCs making this same roll.
First is the capable, wilderness-savvy ranger type. He’s got a special action or feat or something that gives him an Ability of 6 to notice ambushes. He rolls d4 (3), d6 (3), d8 (6), and d10 (2). This reduces the AC from 18 to 15, 12, 6, and 4, respectively. His final roll on d12 (11) brings it to -7, fully paying off the Action Cost and leaving 7 points of Risk. He adds his Ability leaving 1 point of Risk. “You overlook a minor detail,” says the GM. “You’ll find out about it after the fight.” The player agrees, and the combat begins.
Second is the bespectacled professor type mentioned earlier. He’s not old and senile, but he sometimes misses things, and has an Ability of 1 to Realize non-scientific things. If he had rolled the same as the ranger, but added his Ability of 1, 6 points of risk would have remained! Maybe the professor, peering intently into the jungle, is the first attacked, and the signs of the ambush is him falling backward with arrows in his chest. Maybe he’s stunned and unable to move for a few rounds. Maybe he flees in a panic.
The professor’s player could have chosen to stop rolling, and failed to detect the ambush at no personal cost. But either way, the increasingly big gamble of the dice is a player choice, with both big rewards and big penalties as possibilities. If it really works as intended, then success or failure is a choice the player gets a say in, at the possible cost of something going south very hard for their PC.