The principles behind casting a French leader are the same as for standard fly casting. Therefore before a cast is performed the leader must have no slack in it. Therefore the current is allowed to take the leader and straighten it downstream.
The rod is loaded by executing a cast. In other words the rod is smoothly accelerated and then suddenly stopped. In clever language the rod loads as a consequence of the inertia in the nymphs.
The long tapered leader helps propel the weighted nymph where desired. For reasons of stealth on small streams like this this is upstream of the angler. On bigger streams nymphs are fished upstream and downstream and across.
The cast is almost complete. The nymphs are fished a considerable distance ahead of me in the stream. The in-line indicator is clearly visible. These in-line indicators are purely used as a sighter and have no flotation effect like twist on or foam indicators. Modern in line indicators are often short sections of hi-viz nylon with fluorescent coloured beads added.
The rod is held high and lifted as the nymphs travel back downstream to maintain contact. The long tapered leader is not allowed to rest directly on the surface. The nymphs have been selected based on weight. This is the most critical aspect of long nymphing.
This following clip demonstrates all forms of European Style Nymphing including French nymphing. In the right conditions the technique is very effective indeed.