[Konstantinos]

I had similar problem months ago, and it worked only for vehicles. I'm now using something similar to the code above and it works for all the vehicles.

pawn Код:

public OnPlayerKeyStateChange(playerid, newkeys, oldkeys)
{
    new
        Float:vx,
        Float:vy,
        Float:vz
    ;
    if((newkeys & KEY_LOOK_BEHIND) && !(oldkeys & KEY_LOOK_BEHIND))
    {
        if(GetPlayerState(playerid) == PLAYER_STATE_DRIVER)
        {
            GetVehicleVelocity(GetPlayerVehicleID(playerid), vx, vy, vz);
            if(floatabs(vx) < 3 && floatabs(vy) < 3 && floatabs(vz) < 3)
            {
                SetVehicleVelocity(GetPlayerVehicleID(playerid), vx * 1.5, vy * 1.5, vz * 1.5);
            }
        }
    }
    return 1;
}

By Matite (I think).