Fling
Flinging is a basic movement skill in 
Portal (series). The player converts a downwards kinetic energy from gravity of either themselves or an object to the same energy directed straight up, horizontally or diagonally. This skill is performed by placing a portal on a surface far below the the player and another portal facing the intended direction of movement. The player then falls into the first portal, accelerating due to gravity. As momentum is conserved between portals, the player is then "flung" out of the second portal at the velocity they reached accelerating downwards.
However, there are major differences in how these are handled in each game.
Air Control
In Portal, the player can airstrafe during a fling to bend their flying arc, in a similar manner to the Demoman or the Soldier in 
Team Fortress 2. It is possible to enter a floor portal diagonally while crouched to perform special flings and to fling crouched to reach higher places.
Since 
Portal 2, there is little to no air control; it is kept to a slight bending and as such can be predicted accurately or be a result of a prediction. It is no longer possible to crouch-jump into a portal—entering into a portal crouched makes no difference, but flinging diagonally is still a possible move if the portal is placed on a diagonal surface or the player enters the portal diagonally.
Portal 2 introduces the Aerial Faith Plate, which acts similarly to a fling, using a trigger_catapult instead of a pair of portals. It is used extensively in places in which the level designer wants to make the player land on a small area, keeping the "random factor" deaths to a minimum. Debugging tools are available through the ent_bbox and ent_absbox commands to visualize predicted flinging arcs from a trigger_catapult.
Physical Props
Props like cubes are handled differently in Portal and in Portal 2.
In Portal, it uses about the same system as the gravity gun, as the handled prop keeps a lot of its physical properties. Also, it is possible to get killed by colliding with a speeding prop or by flinging into an immobile prop.
Since Portal 2, some of the physical properties of the handled prop are sacrificed to allow the player to fling with those props without too much of a problem. The player takes no damage from physical interaction with props.
