In this context, a knockout ring is a ring surrounding the arbor hole of a saw blade that is partially pre-punched and can be relatively easily broken loose (knocked out) to enarge the arbor hole. Knockout rings are commonly used to avoid having to make and sell blades with different sizes of arbor holes.

In the following reference, it appears that the term is also used for a ring that can be fitted into the arbor hole to reduce its diameter.

PDF] Redzaw Diamond Blades Datasheet
... Designed for consistent cutting performance in a wide range of materials • 1" arbor with knockout ring for 20mm arbor • Corner blade includes 210" width ...
www.redzaw.com/images/ds_rzblades.pdf

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Note added at 28 mins (2005-02-09 14:01:41 GMT)
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Hmmm, it looks like I found the text your\'re translating. I can\'t tell from the document whether blade comes with the ring attached (thus with a 20-mm arbor opening that can be enlarged to 1 inch) or with a ring that can be inserted to reduce the arbor opening.