Tight-line (euro) nymphing is the Blowtorch's home water. Run it as the
point — the anchor fly — dead-drifted deep through riffles, pocket water, fast seams and slightly-stained flows, with a slimmer, more natural nymph on the dropper above it. The logic is contrast: the Blowtorch pulls fish in from a distance; the natural fly fools the ones that come to look. It also fishes well under an indicator, or as the weighted anchor in a dry-dropper rig — see
river nymphing for the mechanics.
When to reach for it
Cold mornings, off-coloured water, broken pocket water, fast seams that need a fly on the bottom now — any time the fish need to be triggered rather than fooled. When they get fussy, downsize it or move the natural on your dropper up in the pecking order.
Sources & further reading: Tim Cammisa, "The Blowtorch Nymph I Actually Fish" (YouTube); the Trout & Feather write-up (materials & rigging); and Tactical Fly Fisher for the pattern's origin. Videos © their respective creators, embedded from YouTube.