The Cardiff Dartheat is especially developed for working the lure down to the bottom. The lure makes a dart action when giving a small jerk. This is especially effective for rainbow trout during winter time, when most fish are concentrated near the bottom due to the low water temperature.
The sharp darting action really triggers the fish to bite. The recommended action is to let the lure sink to the bottom, give a small jerk to let the bait dart and wait around 3 seconds before making the next dart.
The pause during winter can be really essential to let the fish come close, observe the bait, and when you give the next dart, the fish will be directly triggered and grab the bait. Also during the fall the bait has a nice wobble down and the fin on the back ensures a stable position on the bottom.