What determines waterflow from underneath rim?

I have 2 Toto toilets that I installed a few years ago. One is a Drake and the other is the Ultramax.

What determines how much water is released down the bowl during a flush? Sometimes I wish my Totos had more of a cleaning effect by releasing more water from underneath the rim.

To meet the low-flow federal requirements, many use a siphon jet arrangement to get the toilet to siphon and therefore evacuate the bowl. This doesn't leave much extra water to wash the walls. The Totos that use the double cyclone flush can do a better job, but unless the holes under the rim are plugged up with mineral deposits, there's not much you can do to get more water from there. A wash-down bowl does a better job, but that system doesn't work well with the typical low-flow designs.