That is the theory. In practice, unless you are directly above the snag ie. in a boat, the retriever will not slide down the line because of the drag of the water and the line stretching and sagging under the weight of the retriever. If you fish from the bank they are almost useless.
Here is how to make your own retriever that is fairly effective from the bank and very cheap to make.
You will need:
Take the treble hooks and cut off the points just below the barb. The thickness of the hook wire is important. If they are too strong you may get them caught up on some unmovable snag and lose your retriever, but if they will give under maximum strain they will straighten if you hook a sunken tractor or suchlike and you will get your retriever back! The size of the trebles is important, too big and they catch on everything, too small and they won't hook the snag. You should use a file to smooth over the cut ends of the hooks.
The lead weight should be bomb-shaped with a simple thick wire loop attachment, rather than a swivel which will eventually break
An old bulk line spool might do as a cord winder, if it is big enough. Failing that, use a piece of wooden dowel.
Fix one end of the line to the winder and to the other tie the lead. Leave a long tail so that one of the trebles can be tied to it, use a loop knot to tie the other treble a couple of inches above the lead.
When your lure is snagged try to judge the exact position of the snag, taking account of angle of line, depth of water and strength of current. Unwind enough line from the winder to reach further than the snag, then make sure the winder is secure, in a zipped pocket, say. Everything can get very messy now so take care that the line is loosely coiled on the ground and not caught around any foliage.Make sure also that your rod is safely out of the way and not likely to be pulled into the water if the weight drops over the tight line.
Now throw the retriever out just beyond the estimated position of the snag. Allow it to sink on a loose line, then slowly pull it back, allowing it to bump along the bottom. It can take a surprising number of attempts to get even a big snag, but I reckon on a 90% success rate, be patient and persevere.
Occasionally you simply hook the lure and get it back, but usually you get the whole snag and remove it wholesale from the water. You often collect a few items that other anglers have lost, especially swimfeeders, but on rare occasions a lure. Before throwing the snag up the bank take care to remove any line that might trap wildlife.
There is only one hitch with all this, it can be such good fun dredging for sunken treasure that you forget all about the fishing!