I have not been able to find this command on the forum , so I thought I'd paste it here for help.
Add this in onPlayerCommand( player, cmd, text)
if ( cmd == "flip" )
if ( !player.Vehicle ) MessagePlayer( "[#ff1111]Error: [#ffffff]You need to be in a Vehicle to use this command." , player )
local veh = player.Vehicle;
veh.Rotation = Quaternion( 0.0, 0.0, 0.0, 0.0 );
MessagePlayer( "[#00ff00][INFO] [#ffffff]You have flipped your car." , player )
Hope it helps :D