The Buna Spider insert is used to provide a rubber cushion between the two jaw type couplers, preventing metal to metal contact.
The open center spiders included with the Power Grinder grain mill motor package feature an open center design, which allows for the shafts of the driver and driven to
be positioned within a short distance. Open center spiders offer shaft positioning flexibility, and help provide for a small tolerance of shaft misalignment. Although the shafts being coupled should be as perfectly aligned as possible, because the shaft rotation speed is relatively low, an exact alignment is not required. The Lovejoy video linked below shows the process for installing and using shaft couplers. Please keep in mind that unless you are building a space shuttle, or installing shaft couplers on shafts which will be spinning at thousands of RPM, some of the tools and equipment suggested in the video are probably not required.