i think for alot of people, including myself, the concept is quite difficult to understand...i have to double check myself when explaining it too haha, the wording alone makes it confusing. lets see if i can try and get it right this time

in general, when you move your top and lower arms or links closer together (vertically) you make the roll center HIGHER.
I just think off the total amount of "car" that will now be above that imaginary centre line. think about it this way....a double decker bus has a HIGH roll centre because its swing arms are basically on road level and the rest of it is 5m in the air and therefor top heavy...it will fall over easier.

the inverse is true for lets say a supercar which is very low to the ground for example and alot of the actual car is closer to the ground and that imaginary centre line.

but that alone does not determine what happens when we change it. it basically comes down to how the geometry of the car is changed and used to transfer the weight of the car around a centre line to generate side traction around corners.

ok lets try lol
So, moving the top and lower arms or links further away from each other and making the gap bigger (vertically), LOWERS your roll centre because less of the actuall car is above an imaginary line somewhere in the centre of the car.

in general:

FOR the FRONT:

LOWER roll centre (arms/links further apart vertically) = in general makes the car more responsive.

HIGHER roll centre (arms/links closer together vertically) = in general it makes the car less responsive


In the rear it is similar in terms of the above info but a little more complicated because we can play with link arm lenghts as well, changing the geometry a little.

raising the rear link = larger vertical gap between top and lower arm/link
= LOWER roll center
= less rear traction/more steering

lowering rear link on tower = smaller vertical gap between top and lower
arm/link
= HIGHER roll centre
= more rear traction

rear link length has the following effects depending on where you put them on the hubs and tower in the rear:

SHORTER = less rear traction
LONGER = more rear traction

hope that helps...still pretty confusing if you ask me

PS: most of this info is in the HUDY setup book for 1/8 off-road vehicles (somewhere near page 20 i think)
you can download it from xray's website