[Railman]

I would consider that to be a valve lapper, not grinder.
Typically valves are ground on a machine, as are the heads. Then they are fine tuned by lapping. The most common way is to use one of these:

https://www.amazon.com/Performance-Tool-W86552-2-Piece-Lapper/dp/B004GBJAOG/ref=asc_df_B004GBJAOG/?tag=hyprod-20&linkCode=df0&hvadid=312158556601&hvpos=&hvnetw=g&hvrand=5598587738897460072&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9015741&hvtargid=pla-435625204450&psc=1

 

[Railman]

Lapping is done with an abrasive paste, in a back & forth motion, similar to starting a fire by rotating a stick.

 

[Railman]

Hand lapping is done by a back and forth motion plus small rotation when you re position hands on tool shaft.This tool duplicates same action.