You can use this to add a new Opening command before the currently selected command in a shape's command list. (Position of the command in the list will have an effect on its spatial consequence.) This new command will start out with its parameters randomized, but you can change them to whatever you want.
Each shape in a design maintains a variable-length list of operations in an internal language. This list can be modified in mutation to lengthen, shorten, and recombine with others. These commands are run on each shape as it goes from genotype to phenotype.
The Opening command cuts a piece out of a shape. It takes two parameters both from 0-1: start of cut and length of cut. The start of the cut is parameterized around the closed curve from 0 to 1. The lenght of the cut is the proportion of the curve to cut away.
(Opening is notoriously flaky. If you see odd results do not be too surprised.)