If desired, add salt to machine. Adding salt to the dye bath makes the fabric dye stronger because it forces the dye out of the water and into the fabric. Salt does not set the dye. It just makes dye migrate out of the water and into the fabric.
http://www.bestdye.com/machine-vat-instructions.php


The anti-migration agent of this invention is employable in the dip dyeing and printing for both hydrophobic and hydrophilic fibers, and it exhibits an outstanding effect particularly when used in the thermosol dyeing for a polyester cloth using a disperse dye, in the pad dry cure process dyeing for a cellulosic cloth (e.g. cotton, hemp) using a reactive dye, and in the vat steam process dyeing using a vat dye. In using the anti-migration agent of this invention, various additives may be added unless they badly affect dyeing, for example, a neutral salt of an alkali metal such as sodium alginate or common salt.

The amount of the anti-migration agent of this invention to be used varies according to the kind of cloth to be dyed, the kind of dye and the dyeing method, but it is in the range of from 0.05% to 10%, preferably from 0.1% to 5%, of the dyeing solution in terms of the total weight of the neutral polymer salt.

http://www.freepatentsonline.com/4337062.html

La pianta viene usata anche per tingere in verde, grazie a bagni di rimonta con Isatis tinctoria L. (guado) su piede di ginestra.
http://www.retecivica.trieste.it/triestecultura/musei/scient...

La risposta mi è venuta in mente leggendo l'ultimo link indicato. Mi sembra che per bagno di rimonta si intenda un bagno di colore in cui quest'ultimo migra dalla soluzione acquosa al tessuto. Da qui il resto..che leggi sopra.

Bassa confi, non è il mio campo, ma il termine era sfizioso e mi sono messa a cercare qua e là. HTH

according to this link -

http://www.visit4info.com/advert/Stain-Removal-Colour-Run-Tr...

other 2 answers sound OK too