Okay, there is no true explanation for what you are asking. The problem is this-
There was no clear class of FDS until 1935. That's when JW's told the rank and file
that there were some going to heaven and some living forever on earth.
There never was a turnover of power from FDS to GB. The leader was the president
of the corporation, his board of directors was the Governing Body, but they did
whatever he said, until 1975 (give or take a year) when the board voted to take
control over the president.
The JW's cannot explain the FDS giving authority to the GB, because they slowly
convinced them that it has always been that way, when the FDS actually never
had any say-so in any matters.