Why It's an Essential Tool
A chalk snap line is an inexpensive way to draw a long line between two points when a ruler is not feasible.
For instance, you may have a large piece of plywood that you need to rip from one end to the other. By using a chalk line, you can snap a reasonably steady line the length of the board. Or, you might have located the top and bottom points of a stud within your wall. By snapping a line between those two points, you can indicate where that stud is, showing its entire length. It is important to realize that a chalk line does not take the place of a straight edge. The line produced by the chalk snap line is fuzzy and sometimes hard to follow when cuttting along it.
First, shake up the chalk inside your tool (or add chalk if it does not have any). Then, hook one end of the chalk snap line to your beginning point. If you have a well-functioning chalk snap line, you can slowly play out the line without using the crank. If the line plays out with difficulty, then be sure to use the crank to reduce the strain on the line.
Affix End Point of the Snap Line and Unreel
Hold down the endpoint of your chalk line firmly with your thumb. If you are playing out a very long line, then drive a temporary screw or nail at this point and wind the string around the screw/nail, freeing you to move to the center point of the line.
Snap the Line
Make sure the line is firmly resting against the surface. If there is any gap between the line and the surface, then it will not work. Hold back the line one inch as if drawing back the string for a bow and arrow. Let go: that is your snap.
Chalk Line Remains From String
Inspect the results of your chalk snap. In just a few seconds, you have drawn a relatively clear line without resorting to using a straight edge.