Portal Funneling
Portal funneling is a game mechanic in the Portal series. It automatically corrects the positions of players and objects moving at a high speed towards a portal, so that they go perfectly into the portal rather than potentially hit the rim, much to the player's annoyance. It also appears to sometimes affect trajectory after the object goes through a portal. Note that players must be looking at a portal when funneling into it.
In Portal, funneling can be toggled in the options menu. In Portal 2 it can only be disabled by console command.
Related console commands and variables
sv_player_funnel_into_portals Enables funneling.
portal_funnel_debug 
Prints debugging info to the console when funnel code is triggered.
speed_funnelling_enabled
Enables funneling when running on propulsion gel. (Note that the command spells "funneling" incorrectly.)
sv_player_funnel_gimme_dot
When set to higher than 1, player trajectory will never be forced to be perpendicular to the portal after crossing it.
sv_player_funnel_height_adjust
When the player is this far above a portal on the ground, funneling will occur. Below this height, none will happen.
sv_player_funnel_snap_threshold
[Todo]
sv_player_funnel_speed_bonus
[Todo]
sv_player_funnel_well_above
[Todo]
sv_props_funnel_into_portals
Enables funneling for objects such as prop_weighted_cubes.
sv_props_funnel_into_portals_deceleration
[Todo]