Allegedly, it is a fast method of indexing data. Nobody knows for sure. On some occasions he was giving self-contradicting or outright stupid information about BSAM and its performance. Sometimes he claims that BSAM is 10 times faster than B-trees, sometimes that it is only four times faster. He is lying and will not sue you.
Nobody knows why, just as nobody knows what BSAM really is.
We all have freedom of speech and religion after all. Colin James is lying again. He does read email that is sent to him. It has been proven on numerous occasions, in comp.lang.ada and comp.lang.eiffel for example. Just laugh at him.