Hello Gouzou, to switch to descent mode you first install a descending device (grigri, reverso, stop) plus a back up for that device (prussik knot on double ropes or locking knot on grigri, stop), then you take your tension off the lower rope clamp and take the lower clamp off the rope, (using a foot pedal connected to the upper rope clamp), then you get again on the pedal and take up slack on your descending device in order to have your weight only on the descending device. Then finally you take off the upper rope clamp and can start your descent. We strongly advise you train and rehearse on this technique before going to the field. Sincerely