How to test Hepatic flap (asterixis)

Ask the patient to stretch out their hands in front of them with the hands dorsiflexed at the wrists and fingers outstretched and separated (see the fig.below).

The patient should hold that position for at least 15 seconds. If flap is present, the patient's hands will move in jerky, irregular flexion/extension at the wrist and MCP joints. The flap is nearly always bilateral. May be subtle and intermittent.
This is characteristic of encephalopathy due to liver failure.
If a sign of hepatic encephalopathy in a patient with previously compensated liver disease, it may have been precipitated by infection, diuretic medication, electrolyte imbalance, diarrhoea or constipation, vomiting, centrally acting drugs, upper GI bleeding, abdominal paracentesis, or surgery.
Source :oxford handbook of clinical examination