What trick does Roy Cohn play on Ethel Rosenberg at the end of Act 4?

Roy Cohn, in his last moments of breath pretends to confuse Ethel with his mother. He puts on a pathetic display, and Ethel, pitying him, sings him a lullaby in Yiddish. As she finishes, he appears to be dead. Suddenly he springs back to life laughing because after all these years he finally made Ethel Rosenberg "sing."