Do not worry - I got you covered on both counts...
Sometimes, players get really lucky on their rolls. Or, sometimes you just want to present the group with something meant to inspire fear. If you need things to suddenly get more challenging, you would apply one of Hindrances you had pinned for just such an occasion.
But the opposite can also be true: sometimes, players get their pants stolen on a bad roll of the dice (to be fair, for that scene the group was meeting a young master thief who specializes in stealing that which is hardest to steal). Sometimes, you get caught falling into the lion’s den. For those cases where you feel the difficulty of the scene seems to be taking away from everyone’s fun at the table, you can always apply one of the Helps you had pinned for just such an occasion.
Now, this is by no means something as simple as “Description: [Insert modifier here]”. If we had a Scene called “Stop, Thief!” it might look something like this: