Hi Thomas,
It means he's using something that is slower than the 4th harmonic, and wants to see where all his lines end up. Imagine watching a planet transit past NY. That's going to happen one time per day. If you watch the 2nd harmonic of that (add in the 180), that's twice a day, and if you watch the 3rd, that's 3 times per day, etc. 6.5 hour days (day session) is only 27% of a full 24-hour period, which means you need the 4th or faster harmonics to guarantee one line to show during that period. If you're watching anything slower, it's possible that you don't get any lines at all. Usually that would make you think that the particular signature you are watching isn't going to apply on that day, but if you like to slide things around and look for resonance at specific intervals from a point, etc, then you need to know where that line is in order to be able to apply your analysis. The solution in that case is so to include the night session in there (or some part of it), and you can figure out where to start from.
As an interesting aside, I've heard arguments from the pure geometry guys that you need to change the length of the day as well, but in their case a lot of them shorten it, rather than make it longer. Michael Jenkins is a good example of that - he advocates a 6 hour day, basically chopping off the extra half hour in there. In other words, getting to an exact 25% of a day (rather than 27%), makes the multiple-day geometry lock in better.
And as long as I'm on the subject... I've also done a bunch of work (back before W59 existed, actually), that suggested you might be better off only including certain days, rather than all of them. For example, only plotting every 4th calendar day on your chart lets you do some really interesting things with intraday seasonals. Whether or not it's a real phenomenon, or whether it just lets you track the Wells Wilder/Jim Sloman traders is unclear, but I had a pretty slick indicator built based on that sort of thing back in those days. Getting completely off the subject at this point, but just FYI.
Regards,
Earik