A fluid thrill is felt by tapping one side of a lump and feeling the transmitted vibration at it reaches to other side.
If a swelling is large then vibration waves may be transmitted along its wall. Therefore if a lump is large then the edge of the patient's (or an assistant's) hand is placed on the lump midway between where the examiner is tapping the lump and where he is feeling for the vibration.
If the lump is small then it is impossible to distinguish between a fluid thrill and the mechanical shaking of the tissue caused by percussion.
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