I think it was the blood splattering on him. Was he in on it? I thought he was at first but then later when he's talking about it I'm not sure.
I think it's to give an impression of what it must have actually been like for Sil at the time. He didn't know what was happening, for a split second he went deaf and time seemed to slow down. He wondered what the blood was all about for a fraction of a second before he realised what was happening.
I thought it was a very good scene and a pretty unique way of showing a hit.
