Quoted from the download page:
How does Cop Holster work?
All ped models in GTA have what are called drawables. They are pieces of clothing or equipment that can vary between instances of that model. Many of the default cop models have two holster drawables. One is empty, and one has a pistol. Cop Holster simply changes between those two drawables depending on if you have your pistol drawn or not to give the illusion that you are taking the pistol out of the holster.
I’m a ped model developer and would like to make my model work with Cop Holster. How would I go about doing that?
See the readme in the download.