By default the QuimDC doesn't enable its motors. You must enable them before using the board. Use enterOperationalMode command to enter in operational mode and enable motors and control loops.
The board has a watchdog that is decreased every 1 ms. When the watchdog value reaches zero, the QuimDC stops the two motors by sending them a 128 PWM. Each time ref_speed, ref_current or ref_position is set, the watchdog is reset to its original value. A watchdog of -1 means that there is no watchdog counter.