“A floating needle is a rhythmic sweep of the dial at a slow, even pace of the needle.

“That's what an F/N is. No other definition is correct.”

HCOB 21 July 1978
What Is a Floating Needle
(Tech Vol XI, p. 204)

“One does not sit and study and be sure of an "F/N." It swings or pops, he lets the pc cognite and then indicates the F/N to the pc, preventing overrun."

HCOB 21 October 1968R
Floating Needles
(Tech Vol. VIII, p. 258)

As far as how to understand the definition of an F/N (or any other tech):

“All Word Clearing is done under the discipline of the Auditor's Code."

HCOB 13 September 1971
Word Clearing Series 23, Troubleshooting
(Subject Vol. 2, p. 180)