The rapid rhythm in the upper chamber (atrium) cause a stagnation of blood flow through it - it beats too fast to force the blood through it efficiently.
Because of this, the left atrial appendage (laa) is the area where >90% of the clots form in af. Clots in the laa break off and flow out of the heart
and can cause a stroke
. Anticoagulation - blood thing medications - reduce this risk.