at the end of 5a the gemorah has a case of where you caught water in one hand and swatted into the other hand and therefore you did the akirah. the gemorah asks but you need an akirah on 4 by 4 surface. but what is the qustiuon the gemorah said right before that the hand of a person is considered 4 by 4?
Moshe Heller, La,Ca
The person did not catch the water in his hand and then swat it into the other hand. He just swatted the water as it was falling. The Akirah is from the "air" in which it was falling, and not from his hand (which is just being used to swat).