Join Date: Jan 2000
Location: Steelhead country|striper coast|bonefish belt
Trunking can occur at the end of the sweep as the d-loop is made -or- it can be made "along the way". In other words, one case is when the lift and sweep are fine but the rod trunks in the end at the d-loop release (e.g. the rise), while the other is when the rod trunks straight out of the lift in an attempt to create a d-loop.
If the bottom kicks out at the final firing position, the d-loop falls into the water as Dana mentioned above. If the bottom is kicked out during the sweep, or in order to facilitate the motion backward (which I see often) then there is a loss of power and inadequate energy to form a proper d-loop in the first place. It's really the same problem but done at a different point in the stroke, and requires an additional different remedy.
Trunking pivots the rod about a single point between the hands instead of holding an arc in the rod until the point of energy release into the d-loop. A pivot can only put the rotating power of the rod tip into the d-loop effort, not enough to replace the sweep. When bringing the "load" backward toward the d-loop, keeping the bottom hand close to the body and in proximity of the other arm throughout the sweep ensures that the proper tension is kept in the rod blank until the d-loop can be formed.
The exception to this is underhand casting with short heads and a light anchor. Short lines are much more forgiving and can physically be set into a d-loop with just a pivot. In fact some underhand casters appear to be trunking, just before the cast rips out over the water in a tight loop.
All other lines, especially extended belly lines require a lot of attention to proper and full d-loop formation without any trunking whether in the sweep or the final firing position.