Urinary stress incontinence is the involuntary release of urine, triggered by an increase in pressure on your bladder.
As your bladder fills, the pressure inside increases and it becomes harder to hold on.
In addition, many activities create a big enough increase in pressure in your abdomen to trigger a leak: involuntary movements like coughing and sneezing; vigorous actions like running and jumping; or even slower movements like getting up from sitting.
Your body is very clever and is normally able to both provide constant support to prevent your bladder from leaking as it fills up and to react to any sudden increase in pressure. Muscles squeeze your urethra (the tube where the wee comes out) tightly at the end to prevent it leaking - any increase in pressure requires a corresponding increase in support.
Unfortunately, many women lose their ability to respond effectively to this pressure and can't hold it in.
Your urethra is supported and held closed by your pelvic floor muscles. If they are fit and healthy, they will provide the constant support needed to cope under pressure. When you go to the toilet, you let go of that muscle tone and your bladder empties through your urethra.
Your pelvic floor muscles are "always on", they have enough tension, or "tone", to keep the urethra closed so that nothing can escape. They need to do this all day and all night - it's hard work and they need to be fit for this purpose.
If they are not up to the job, as the pressure builds, you risk a leak.
In addition to this constant work, if pressure in your tummy goes up suddenly (for example, because you move, jump, cough or sneeze), it presses on your bladder creating a sudden and extreme increase in pressure. Your pelvic floor muscles need to react to this - they need to quickly tense up, closing the urethral opening more firmly.
If your pelvic floor muscles have become weak, they are no longer able to provide the necessary support (either quickly enough or strongly enough) and you will leak.
You leak because your pelvic floor is not working properly.