Bruces war cries, how did he do it?

Bruce lees war cries in "fist of fury" have never been better. Indeed in certain scenes they are quite frightening and echo the insanity and anger of Lee's character Chen Zen.
Does anyone out there know of any articles that have been written specifically on how he went about dubbing these sounds. I would imagine it would have been a strange sight watching the dragon screaming into a microphone while watching himself on a studio screen. "kia" is an art form in itself and requires breath control and vocal power.It was rumoured in Hong Kong that the energy used by lee to dub these sounds actually caused his brain to burst a blood vessel.Note the collapse he suffered on May the 10th 1973 while dubbing "Enter the dragon"