The advantage of lead knives over lead dykes are that a wider variety of angles can be made with the knife. There are a number of varieties of lead knives - ranging from adapted paint scrapers to specially made sophisticated tools.
The technique in using a knife is to wiggle or rock the blade with some moderate pressure down through the lead came. Excessive pressure will twist the came rather than cut it cleanly and squarely. When you find the effort or time required to cut through the came has increased, it is time to sharpen the blade. The sharpening angle should be very acute. You can use a fine oil stone or wet and dry sand paper to hone the cutting edge. You can also use a little bit of wax or soap on the blade to ease its passage through the came.
The most simple knife is a stiff paint scraper. The blade should be of good steel so that it takes and retains the sharpening that is needed from time to time. This blade works best by wiggling through the came.
Another style of lead knife has a curved blade. This has a number of variations. This knife works both by wiggling and by rocking.
Experience and personal preference will determine which style you settle on. The important elements are to make sure it is made from good steel and that it fits your hand comfortably.
You can add a metal end to the handle to provide a tool with more uses, especially as a hammer to put the nails into the board, or to snug up wooden borders, sometimes even gently tap the glass into place.
Also look at the use of lead dykes.