• Atrial fibrillation affects your heart's ability to pump the blood normally. This causes blood to pool in the LAA (left atrial appendage) portion of the heart, at which it forms a clot.
• A blood clot can migrate from the LAA to other regions of the body, cutting off the brain's blood supply and resulting in a stroke.
• In people with atrial fibrillation not caused by heart valve problems, more than 90% of the strokes are caused by the clots that come from LAA.
• Watchman device works through the principle of occlusion.
• The Watchman device fits right into the LAA and permanently closes it off and prevents those blood clots from escaping.
• Over time heart tissue forms over and around device permanently sealing off the opening thus preventing any blood clots to enter into it.

Watch this video to learn how watchman works 

Reference - https://acsdoctors.com/patient-info/acswatchman/