I'm not sure what you mean by "arc", but I think you mean the steering axis?
Image the top of the strut, the centre of the hub and the point where the tyre contacts the surface - most cars will have a vertical line through this and this is the steering axis. The kit creates a postive axis by moving the hub forwards so the vertical line is now an angle, so the steering axis is slightly in front of the point where the tyre comes into contact with the surface. This creates caster, so the wheel follows the strut like a shopping trolley's does. What it also does is give dynamic camber, positive on the inside wheel and negative on the outside wheel, so it is true in a straight line and the more you turn the wheel, the more camber you get.. so not so useful in fast sweepeing bends, but very useful slow tight ones because of the 'footprint' being bigger and the 'effective' downward force on the inside wheel.
A way to think about it, imagine a unicycle, you turn it and it spins on the wheel through a vertical axis and achieves no camber.. now imagine a chopper with long forks, you turn the handle bars and it creates a lot of camber, but runs none in a straight line.
It can have a negative effect in extremes, in that it can make steering harder and because some of the lateral force is taken from the struts, the suspension actually works better... this can however be seen as "dive" by some. But thankfully on a MacPherson strut it's quite difficult to go too far. Someone also explained to me that go-karts run a lot of caster, so when you sit on them and turn the steering wheel it'll go on 3 wheels as it pushes the inside one down, but that's also why they have a very heavy steering action. Obviously a car won't do this as it has suspension travel and a lot more weight! - but a good visualisation of its effect.
Does that help?
