A dyeing process to maximize colour penetration. It also makes the yarn keep its soft feel. Yarn is looped over a hook and dipped in water, which opens up the fibres. This allows the dye to reach everywhere.
The fabric is left in the dye for 48 hours, then washed and redipped, a process that is repeated a few times.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary