Each leaf represents a complete spring in it's self and will act as such. To enable this the leaf is tapered, from the center (thick) to the outer ends (thin). This tapering is parabolic, it means that every centimeter (or inch) the thickness of the leaf decreases in an amount that relates to the square function of it's length.
This may sound complicated but in fact it is very simple. Every single leaf will have, more or less, the shape of a complete multi leaf spring and thus it's capable to cope with the same forces. This means that the ideal parabolic spring could have only one leaf, however, this type of "mono leaf" spring will have very limited articulation/weight ratio due to high internal stresses so a 2 or 3 leaf parabolic spring can divide the stresses more evenly across the other leaves and thus more axle movement is possible. That is why we designed 2 and 3 leaf springs. An other reason to increase the number of leaves on a parabolic spring is to increase the rate (load capacity). Because every leaf is one spring we can ad or take out leaves without compromising the strength of the leaf it self. So for example we have a 3 leaf rear spring and we ad one similar leaf the rate will go up for about 30% which means that the load capacity increases by +/- 30% (depending on the shackle effect)