OK first things first- A Czech nymph is a fly pattern. Just because you are fishing with Czech nymphs it does not mean you are Czech nymphing. Czech nymphing is a fly fishing technique the basics of which are explained here.
Czech nymphs are small slim weighted flies that are generally representative of many free living Caddis larvae found in streams. The introduction of Czech nymphs took the river fly fishing world by storm. In the 1989 World championship on the Dee nobody could even get close to the Czechs and Poles. Their big secret was short line fixed nymphing and invariably they were using Czech nymphs.
Short fixed line contact nymphing techniques had their derivations in Poland as indeed did Czech nymphs. The first Czech nymphs were tied with sausage skin. In fact various strange materials have been incorporated including Mackerel skin. It was the pragmatic Czechs who took the idea of Czech nymphs and produced durable fishing flies. I believe latex gloves from a Czech team members Wifes Beauty salon were used for the production of the first Czech nymphs.
The following clip shows a Czech nymph tie from one of the victorious World conquering Czech team, Jan Siman.
Most anglers these days prefer bead headed flies to Czech nymphs. There is no doubt however that Czech nymphs remain an effective fly for catching trout in streams.