What is a "subvalve" or also know as "low speed valve?"
A subvalve more effectively meters the oil flow thru the bypass circuit of a compression valve assembly.  Most often the oil flowing thru the bypass circuit is only controlled/adjusted by changing the setting of the compression clicker.  While the compression clicker functions well in altering the total volume of oil that can flow thru the bypass circuit, there is still room for improving how to control/manage the oil flow (pressure) thru the bypass circuit.  A subvalve adds the ability to control the flow of oil thru the bypass circuit with suspension shims AND the clicker adjuster.  With the addition of shims, the oil is no longer able to freely flow thru the bypass circuit without light initial resistance.  This light initial resistance is key as it allows for more effective control of the fork compression damping system thru the entire suspension travel at all speeds/pressures.  Subvalves (aka low speed valves) are common in high end A-Kit forks and all production KYB SSS forks and Showa 49mm twin chamber forks.