Bloodmallet:
Not sure there is a “something out there that explains how to use BloodMallet” but I will try 
The creators of the site may have a Discord thingy or Reddit post that explains this all better than me, I am just not aware of it.
note this is my understanding of how that site works, and how I use it. errors of ignorance on my part are not intended 
when you first load the site and select Havoc as the Demon Hunter spec, the following is selected in the Choices section on the site:
Also of note Bloodmallet is using the following TALENT selections as the BASE for all Demon Hunter Havoc DPS info: Talents: 1 3 1 0 2 2 1
There are two items on the TOP row of the Selections that you should be referencing: Trinkets and Azerite, the other two are meaningless (at least for me lol)
Once you select “Trinkets”, the row beneath is “Patchwerk”. the trinkets for Patchwerk (Single Target) are then listed in depending order which perform the best, each bar indicates the ilvl of the Trinket you may have (hover your mouse over the bar for that Trinket) 370; 385; 400; 410. Each with the DPS associated with that ilvl.
If you select “HecticAddCleave” believe that is AOE damage (and Trinkets remains selected) you will then see the same info as described above but instead of ST DPS its now based on AOE damage. (hover your mouse over the bar for that Trinket) 370; 385; 400; 410. Each with the DPS associated with that ilvl.
Note (for below) all the DPS is predicated on the Item lvl being 415 so if your Azerite pieces are 385 (390) the DPS you will see, will not apply to those 390 or 370 pieces to that great amount of DPS, but its still a good gauge for the Traits that will do the best.
Now for the Azerite Traits section, top row, click on Azerite, this will add a different selection row beneath the top row that was not there for Trinkets. Trait stacking; Itemlevel; Head; Shoulders; Chest.
Lets deal with Trait stacking at this point only for now. Select it.
the row beneath that is the Patchwork & HecticAddCleave row, for now select Patchwerk.
Beneath that row is the row with Tier 3 & 4 or Tier 2. Select Tier 3 & 4. You will now have the following sections (buttons) highlighted:
Azerite
Trait Stacking
Patchwork
Tier 3 & 4
(disregard Advanced Options)
*Selecting Tier 3 & 4 - This is for the Gear pieces and the two outer rings only (for those pieces with 4 rings + the final inner upgrade to ilvl.
What you see is the best Azerite Traits to stack in descending order from higher DPS to lowest DPS and the Bars for that Trait tell you how much DPS 1 trait will do, 2 Traits will do or 3 Traits will do for Patchwerk (Single Target)
that is not to say you should just be stacking 3 of those traits exclusively, as others have mentioned, a combination of different Traits is better that only going after one particular Trait exclusively
Now if you then click on Tier 2 that is the third ring in from the outside, traits like Overwhelming Power, Gutripper & Azerite Globules etc. they are displayed again in descending order highest to lowest DPS. Shows how much DPS 1 trait will do, 2 Traits will do or 3 Traits will do
All of the above was Azerite, Patchwerk Trait stacking.
You can now also change from patchwerk to HecticAddCleave for AOE, looking at Tiers 3 & 4, as well as Tier 2 traits to see which is the more effective traits to pursue for AOE focused builds.
For me that is the easiest way to use this site. I suggest stopping there, get comfortable with understanding what you are seeing with what I have described so far. There is more to the site, digest it after understanding Azerite Trait Stacking, Patchwerk or HecticAddCleave information. Once you do, the rest below will make more sense.
Back to the second Row beneath Trinkets & Azerite selections…
You can view by selecting Item Level (with Patchwerk / Tier 3 & 4) still selected, how much DPS a trait like Treacherous Covenant (others at less DPS are below) will do at different ilvl, 370; 385; 400, 415 et cetera by hovering your cursor over the bars for each Trait listed. Same thing for Tier 2 Traits.
Selecting HecticAddCleave gives similar info on AOE traits that perform best and the DPS per ilvl.
On that same row with Item Level you can also look at the Head; Shoulders; Chest . This lets you know what individual gear piece to try and farm or go after. I don’t refer to it much because I am not doing highest content stuff in game, but the info is there to be looked at 
For instance Aerite, Head, Patchwerk selected shows you the Highest DPS Head piece is IronTide Captain’s Hat hover your cursor over that gear item in the list, shows which traits are selected on the piece. The Bar then indicates the relative DPS based on ilvl: 370; 385; 400; 415.
There is NO Tier 3 & 4 or Tier 2 selections in here because those Traits are already preselected on the Gear piece.
Switch to HecticAddCleave for a similar list of gear focused on AOE Traits.
Repeat for shoulders and chest.
So yeah, thats my understanding of Bloodmallet and how I use it, I have to get back to real work now, sorry if I missed anything or just ran out of time and steam. Hope it helps your understanding of how to use the site better.