cmd_velocity

command: /velocity

quick ref:

/velocity [add|set] [@playername] [|x] [|y] [~|z]

description:

add or set velocity to a given entity. 
@param [add|set]: default to set. please note you can not add motion to focused entity, use set instead. 
@param x,y,z: if only one value is provided it means y. if only two values are provided it means x,y.
Examples:
/velocity set @test 1,1,1   :set speed of the test entity
/velocity add @test 1,~,~   :use ~ to retain last speed.
/velocity 1,~,~   :set current player's speed