Taken from the gmod.org description
Changes the players movement to set momentum into play, Building up speed takes time rather than it being instant.
It takes time to build up speed, Not to long, but enough to notice.
Walking/Running up hills/stairs will slow you down.
Running down hill/stairs will speed you up.
Air Control system, You can no longer slide on slanted edges and the such.
Z_SpeedBuild_Enabled 0-1 – Should the momentum addon be enabled?
Z_SpeedBuild_Min NUM – How much speed should we gain from idle to walking each frame.
Z_SpeedBuild_Max NUM – How much speed should we gain from walking to running/sprinting each frame.
Z_SpeedBuild_StartSpeed NUM – What speed should we always start on?
Z_SpeedBuild_Pain 0-1 – Should we be slowed down a little if where hurt?
Z_SpeedBuild_HitGround 0-1 – Should we lose speed if we hit the ground to hard/fast?
Z_SpeedBuild_AirControl NUM – How much air control should we have? Lower numbers mean its harder to control yourself in the air.