The ball joints enable you to use standard tension springs on your Rostock Max V3 or Orion delta. The standard plastic springs are very stiff, and somewhat brittle - some users have reported that the tension can affect arm stability at the edges of the build bed. This mod eliminates the need for replacement springs, and reduces the tension at the outer extremes of build volume. You will need to build 12, and buy 6 looped end tension springs separately.
These are best built using an SLA/DLP resin printer, or a fine nozzle for filament printing.
Big thanks to those of you who have found this Thing useful and have sent tips to me, they are very much appreciated.