To be honest, sometimes a rock worm feels like cheating because it's fishing a worm. It is a fantastic pattern that I've done really well on this winter.
It's best to try and get it when the river's coming down a bit. When it's warm enough for the fish to move but not warm enough for the snow to melt, the fishing can be fantastic.
The main principles for trout fishing are food and cover. If fish can get under the (ice) shelf and feel protected, they will eat more aggressively.