The matter of split contrast filtering is applicable to black and white variable contrast materials. It does not apply to fixed contrast (graded) black and white papers.
With variable contrast materials there are two separate and distinct characteristic emulsions. One is primarily affected by yellow or green and the other is affected primarily by magenta or blue. (The reason that I listed pairs of colors is that some variable contrast light sources use subtractive and some use additive colors. In the case of Lee's aristo cold light head he filters with blue and green. In the case of my Saunders I use magenta and yellow. Magenta and blue are the colors that affect high contrast and the colors of yellow and green affect low contrast.
Normally printing on variable contrast paper is done by varying the ratio of one light color to the other and this arrives at an infinitely variable grade (within the confines of the papers limits).
In the case of split contrast printing the printing is separated into two distinct exposures with one being a soft contrast exposure and the second being a high contrast exposure. One of the advantages of this type of printing are that burning and dodging is more controllable within the two exposures. In other words if I wanted to dodge a shadow area, I would due this during the high contrast exposure since this is the exposure that affects primarily the low values. If I wanted to burn a highlight down, I would do this during the low contrast exposure since that exposure affects primarily the lighter tonal values. Additionally, I feel that I am able to arrive at the correct contrast on the print more rapidly then if I am choosing a single contrast setting.
To summarize, instead of printing a negative at, for instance, 20 seconds at grade 2.5(in a single contrast setting on variable contrast paper)...I may end up doing a 10 second exposure at grade one and a second exposure of 8 seconds at a grade 5.
To expand on Les's explanation. The optimum (maximum effect) of the yellow filtration is 70 units, in his experience. He is saying that to add 120 units, for instance, will do nothing more then 70 units except to add density to the light path and therefore increase printing exposure time.
Hope that this explains this for you. Please feel free to question if you do not understand.