What is overflow incontinence, and how does it occur and get treated?

It depends on causes. Overflow incontinence generally occurs when the bladder is filled beyond its capacity. This frequently happens when the bladder doesn't empty or empties incompletely. It can be due to obstruction (like the bladder falling or from a prior surgery) or damage to the nerves & muscles which cause the bladder to contract. It is frequently treated with intermittent cauterization but there are other options.
Bladder overfilled. Overflow urinary incontinence can occur when the bladder has filled beyond capacity and the intravesical pressure exceeds the closing pressure of the urinary sphincters. Occurs most commonly with neurogenic bladder dysfuction where nerves to sphincters or nerves from bladder are damaged or missing as in spina bifida, MS or spinal cord injuries. Best treated by intermittent bladder catheterization.
Overfilled bladder. That does not empty out at all or only partly . So like a full cup that fluid continues to flow into it run over. Medications, fallen bladder, nerve damage, obstruction are a most of the causes. Treatment involves figuring out which and addressing it.
Overflow? Overflow is an older term that is not used much anymore.. The thought is there is either a partial obstruction as seen with prostate enlargement or after an incontinence procedure or there is a decrease in the bladder muscles's ability to squeeze the urine out as is seen with a lot of neurogenic problems. It usually is a combination of both issues.