Happens automatically, whenever you export an asset from Sketch using the File › Export… menu option or the Export button in the toolbar. The Plugin will run your PNG & JPG assets through
jpegoptim using the quickest settings for both, so that you get smaller files as fast as possible.
Happens when you choose the Plugins › Sketch Image Compressor › Export All Assets menu option. You’ll be asked for a path where your assets will be exported, and then the Plugin will export every exportable layer from your document, and run the assets through
jpegoptim, using more aggressive settings than the Quick Compression (i.e: the operation will be extremely slow).
Please note that both methods won’t block Sketch’s UI when running, so you’ll be able to keep on working while the compressors run. However, running a Full Compression on a non-trivial document will most likely consume a lot of CPU for a long time, so don’t expect Sketch to be as snappy as usual.
You’ll get some feedback about the process in the Sketch window while it’s running, and some stats when it is done.