Well, as the technique is widely used, and I happen to have a very healthy batch of JBP seedlings growing, and being summer around here, and being the seedlings little trunks beginning to turn purple/reddish, I have tried it!
Took some seedlings out of their nursery pots, and with a sterilised scalpel cut all the roots off, making the cut right where the stem? stalk? turns to light green/white.
Planted in pure coarse sand around, and fine sifted sand in a centered cylinder.
It's been three days since, and so far so good.

What is the reason for doing this? You have a seedling with roots, cut off the roots hoping they will grow back, but you had roots to begin with. What am I missing?

Ivan wrote:

What is the reason for doing this? You have a seedling with roots, cut off the roots hoping they will grow back, but you had roots to begin with. What am I missing?

I've done the same with about 15 seedlings I had. You cut the long tap root, but you leave as many as possible of the finer roots. This way you make these finer roots grow, to get a root flare.

Clicio took a more drastic approach, and a more risky one - he cut off all roots. If that works out, you get an even better root spread - better future nebari.

Would be interesting to see this develop.

Auk wrote:

I've done the same with about 15 seedlings I had. You cut the long tap root, but you leave as many as possible of the finer roots. This way you make these finer roots grow, to get a root flare.

Auk is exactly right.
There is a very basic step-by-step instruction HERE.
Thanks, Auk, I will update this thread from time to time.

Found a better explanation of this technique on the link below:

bonsaitonight.com/2011/07/01/how-to-crea...japanese-black-pine/

An other advantages for shohin is that the buds start much lower on the stem.

After one week, not growing, not dying.
I guess roots are forming.

Crossing one finger. And crossing another!

It took about 8 weeks( maybe more) before there where roots growing, when i tried it with pinus densiflora seedlings.

I did that last year with 72 jbp seedlings. I took 4-6 weeks before roots sprouted and the seedlings started to grow again. 4 of them didn't make it

Ofcourse I documented all of this nonsense
shumibonsai.wordpress.com/tag/japanese-b...pine-from-seed-2018/