I was wondering if any of you guys have tried this? Figured it would be interesting to see if it shoots farther or if you get any better “feel” from it.
I have done the opposite, you can overweight a line to add distance to your cast but I dont think it would work if you undersize your line
Line won’t be heavy enough to fully load the rod, resulting in shorter casts.
That’s probably not going to do anything good for you. People say you can overweight by one size. Don’t go under if you can avoid it.
I am currently throwing a 7wt over here with 8wt line on it and I am getting about 10-15ft more on my casts, but I agree with the consensus that you’d get much shorter casts goin a weight ower than your rod… I was told going up one weight in line for your rod will help with longer casts but any more and it over loads the rod and causes problems… Nabbed 81 Fish for February… had to travel a bit so I didn’t get to fish EVERYday but I started off March with an even dozen on a dragon fly nymph I tied up and all were on the over loaded 7wt… finally found the hole they like to hang around at but I couldn’t reach it with the 6wt… 
“Paddle faster boys… I hear banjo music!”
Charleston Director- SCKayakfishing.com
Tarpon 160os
you can actually underline by some. by underlining you are forcing a tighter loop to load the rod, which can help in a breeze. by doing this also you can get away with a little less spooky presentation. less splash on a 6wt head compared to an 8wt. Overlining can also help in the wind but the concept is different. a heavier head is “pushing” more to release from the rod whereas a lighter line is cast through a tight wind cutting loop. i saw this a while back featured in a Florida sportsman, i believe. i tried it and it makes sense and does work
jedi, when da hail ya back?
Hopefully end of April… Thats the first I have heard of underlining a rod… But then coming from you I beleive it Can’t wait to terrorize the ville when I get home… I’ll let ya know when I get back. I get 10 days off so I plan on fishing A LOT
“Paddle faster boys… I hear banjo music!”
Charleston Director- SCKayakfishing.com
Tarpon 160os
I was casting an 8wt rod today and was not happy with the results. When I changed spools to a 9wt line, the rod was completely different. The line shot through the guides and pulled line off the reel.
It can make a huge difference. Now, if I can just stop that tailing loop? 
Chris