Mind you, this was back in the mid-'70s. Bosworth, at that time, basically said that a string would 'relax' about 80-90% of its potential to 'relax' during the first 6 seconds that tension was applied. It wasn't so much the absolute numbers involved as it was an approach to consistency. IF a stringer-person waited 6 seconds between pulling and clamping, A) The effect would be more consistent compared to pull-clamp and then pull---------clamp, demonstrating the need for consistent timing...and 2) For those who pulled twice in those days--somewhat common with Ektelon type cranks--the effect of waiting 6 seconds between pulls would allow the string to 'give up' more of its stretch compared to the rapid pull-clamp approach. Sort of a 'pre-stretch plus' approach.
I'm sure his method has been refined and amended over the 3+ decades since his business became so popular.
We also spoke about his 'reverse-proportional' stringing method; by which he LOWERED the tension in the center mains and crosses to enhance the 'cupping' effect...particularly for Ken Rosewall's Seamco rackets during one particular tournament. I rolled my eyes at that one a bit; but , he swore that Rosewall gave him 'much' credit for bringing his down-the-line forehand back inside the court...thus allowing him to win a particular event.
When stringing for the Atlanta Thunder almost two decades ago, I had the 'privilege' and challenge of stringing one of Martina's(the original) frames to try to match the original Bosworth studio tension. The racket had been in MN's bag for only three days but she thought it might have lost tension over that time! I felt a bit of pressure but MN said it hit just fine--too bad I put the YY stencil on 'reversed' and she could only use the racket in practice, NOT on TV. The YY folks wanted the logo facing the SAME way on every racket. HEY, 'we' still won the World Championships that year!
