Ok, that makes me feel better that I do understand the fundamental concept of the circuit.
I put everything together and found some success using the SD/DT pin to turn off one of the three drivers in each phase. Instead of using a single resistor and connecting all the SD/DT pins together, like in the circuit, I put an individual resistor on each driver to ground. Then I connected digital I/O pins from the uC to each SD/DT. When I want the driver on, I change the digital I/O pin to input mode, which puts it in a high impedance state. That effectively makes the digital I/O line non-existent to the SD/DT pin. When the I want the driver off, I change the the digital I/O pin to output and drive it low. That overcomes the resistor and drives the SD/DT pin low, which turns off the chip.