Frenzy is a melee skill, Double Throw is ranged.
Damage taken goes to mana generates mana based on damage you receive. If you take 10 damage, you will also gain 5 mana in the Nightsmoke example of 50% DTM.
Check out Sigon's Gloves with what you have, you might like them more than the Bloodfists.
More points in Double Swing reduces it's mana cost, eventually to zero (not sure about in 1.09, it's been years, and I'm too lazy to time machine it, but the more I think about it, the more I think this is a 1.10+ feature...)
Off weapon+min/max damage is amplified by skill damage, from sources such as Str/Dex, ED% from attack skill, off weapon ED%, and any other skill based ED% (such as from an aura.)
Thanks @logoutzero , that's useful info. When I got to 24 I put on frenzy and I was able to use my weapons with it without throwing so that was good. I also put on bonesnap on switch and did players 64 meph with it on from lvl 26 to 30. All of the uniques in Durance level 3 just dropped since I had 40% CB on bonesnap + 25% on goblin toe.
After I got to level 30 I equipped two bloodletters and that damage and speed are so high with the frenzy that I was able to leave players 16 and beat act 4 with it on. I'm in A5 ATM, but should be easy enough. Even without all the mana Leech, it seems that even a small amount of Mana Leech from 2-6% seems to take care of that. I probably have like 30%~ + LL ATM and other gear, since each bloodletter gives me 8% LL, then I have 2 Cathan rings , each with 6% LL. With frenzy being level 7 and all of that Leech and damage, and then I have the critical stories always kicking in, Barb's effectively destroyed 1.09. I'm expecting nightmare and hell are just going to get easier given the better equipment .
@fearedbliss Check out posts by @RobbyD, he did extensive 1.09b research/playing and provided a wealth of knowledge to the SPF. I was playing 1.09b & 1.09d back then, but due to computer issues in the intervening time, I've parked that project.