In wood finishing the term “pickling” is generally used to mean adding a white coloring to the wood. (“Liming” is sometimes used to mean the same thing.)
There are two broad methods of pickling. One is to wipe on and wipe off a white stain. The other involves sealing the wood with a first coat of finish, then wiping on and wiping off a stain. There is a big difference in the appearance you get.
In the sample shown, a white stain was applied directly to the lower half of the panel and wiped off. The same white stain was applied over a sealer coat on the top half of the panel and wiped off. The sealer coat prevented the stain from coloring the wood as thoroughly. The white coloring was left just in the pores when the excess stain was wiped off.