A flick, even though a very hard throw to master is the most effective throw anyone can use. Since everyone forces flick in lower levels of ultimate, it is necessary for everyone to be comfortable when throwing a flick under pressure.
Just like how backhands are thrown, flicks (or forehands) are usually thrown parallel to the ground. Two distinct variations of the flick throw are the IO (inside-out) flick and the OI (outside-in) flick. These variations of the flick are thrown when there is a defender in the way of the cutter and the thrower needs to throw it around the defender.
When throwing an IO flick, instead of having the disc parallel to the ground when it is released, the bottom of the disc should be pointed downwards and towards the feet of the thrower. To make the curve more apparent in the disc's path, instead of having the forearm parallel to the ground, the forearm can be pointed a little bit downwards.
The more perpendicular the disc is to the ground, the greater the curve of the disc when it is travelling to the cutter. For an even greater effect, the movement of the arm can be thrown like an underhand throw in softball (although one should not need to do this). Practice at different angles when releasing the disc to understand the effects on the path of the disc.
When throwing an OI flick, instead of having the disc parallel to the ground when it is released, the bottom of the disc should be pointed downwards and away from thrower's feet. The arm can be swung like a regular flick but it can also be leveled higher to shoulder height to make it harder for a defender to reach out and block the disc. Naturally, when the arm is at shoulder level when flicking, it is harder for the wrist to set the disc parallel to the ground as it will have to bend backwards more. Naturally so, the disc will be at an outside-in angle.
The greater the angle of the disc (or the more perpendicular the disc is to the ground), the greater the curve of the disc. Continue practicing
To help players remember the difference between an IO and an OI is to remember that the bottom of the disc pointing inwards towards the feet is an IO (inside-out) and when the bottom of the disc is pointing outwards away from the feet, the throw is an OI (outside-in).
Another way to remember is at the beginning of the throw, if the disc is being thrown towards the body at first, it's an IO. If the disc is being thrown away from the body in the beginning, it is an OI.