I heard some people said, during break-in period, add 0.1 shim to break-in.
When break-in period complete, take away 0.1 shim.
I wondering, is it related to increase the pinch zone area?
Anyone tried and how is it? thx!

No, this has more to to with the compression. The amount of compression is determing the ignition timing. Because with a lot of pinch the compression is higher than when the engine is runned in and the compression is only sealed with the layer of oil between piston and sleeve.

Exactly. Adding the 0.1mm shim only decreases the compression so that the engine becomes a little easier to turn over and get started.
In the beginning the pinch would be at its tightest, so that 0.1mm just makes things a little easier because of the lower compression. Once the pinch lessens, then take out that shim so that it runs the way it should run, unless it just so happens to perform better with that added shim on the track you run at.

No, the easy turn over is not the issue, it is the ignition timing making the engine running more stable and it also saves the rod a bit because a later ignition.

Got it. retard the timing right prevent knocking during break-in right.

well technically i don't get itbut all my engines i add 0.1mm shim. for running in and leave it there coz it performs good enough

From what I have seen is that the compression with some brands is very critical finding the right adjustment without blowing plugs, adding one shim does make that much easier.