I'm no doc, but I am an engineer and I've had the device for a few years now, so here we go...
With a normal urinary system, your sphincter(s) can only hold back a certain amount of hydrolic pressure, no matter how hard you try to hold it in. The AUS is designed to hold back a little less pressure than normal. I don't recall the percentage difference from normal. This insures the bladder and your other important stuff down there isn't damaged from too much pressure. In case of a malfunction, the device also is designed to fail open, holding back no pressure at all.
So yes, if enough external or internal pressure is applied to the bladder and AUS and the is no more space for the urine to go and exceeds the design resistance of the cuff, urine will pass by the cuff until the pressure drops below the cuff's capability to hold it back. Then, flow will stop. A very safe and simple design. Other than that situation, I have virtually no leakage.