The mod is "[0.22] UbioZur Welding Ltd. 2.0"
http://forum.kerbalspaceprogram.com/threads/38577-0-22-UbioZur-Welding-Ltd-2-0-Playtest-5-Now-In-Game-ToolIt says .22 so I would be dubious on if it works properly on 23.5, but it seems to be allright.
The way it works is similar to how sub-assemblies work, you grab a stack of parts and drag it over to the icon which says "Weld it" and then after a minute a window pops up asking what you want to name it and what category you want it placed in.
A few things I have noticed;
A) Welded parts can only be placed from their center, they lose edge connection capability - Careful placement of girders or whatever may be required to get large pieces attached where you want them.
B) Welded parts
only attach at the center, the edges stay unconnected to whatever you shove them onto - Judicious use of struts and extra panels will be required to maintain rigidity of the complete craft.
C) Attempting to weld together parts that have already been welded may result in incorrect orientation of parts - Plan carefully ahead on what parts you will be welding, you only have one shot.
D) Welding breaks fuel lines and struts - Add them after the weld
F) Welding breaks most complicated components - i haven't tested them all, but I have found the following to not work properly after welding:
-Fuel tanks
-Solar panels
-Control surfaces
However when I welded that 747 I linked to earlier in the thread, I found the engines still worked, and the Part still had a cockpit, reaction wheel, etc.
G) Welded part appears indestructable - every crash test shows the complete craft disassembling at the struts and connecting girders, whilst the panels just tumble wildly and settle to the ground.
H) The part retains all preexisting connection nodes - For instance my panels still had the node in the center of each 2x2 it was made from; 121 per side
I) The game seems to treat the part entirely as one object for purposes of physics - There was no noticable lag when using over 700 welded parts, usually a craft of a few hundred causes massive lag for my pc.
J) The part may have stability issues during timewarp - In one test the cube exploded at x2 warp whilst at full throttle in atmosphere, however It was fine at very high time warp while in space; further testing is needed.
K) Attachments to welded parts may be weaker than you expect - I had engines and fuel tanks falling off the cube when attached using large docking ports, a couple of struts held them in place.
TLDR; Use the mod to join large structural or cosmetic parts, then build the rest of your rocket around it
Edit: My next test will involve daisy chaining several cubes together to see what's necessary to join very large items successfully.
Eidit: Actually it is possible to edit the part by opening it's config file and altering it's properties, you should be able to get it's edge connection working again and most of it's other properties too, but I had no success when i tried, the forum might help.