For most reef diving applications a "single" crimp is adequate. But for large or powerful reef fish and blue water fish you might consider the "double" crimp method to ensure your line will not slip through the crimp/sleeve.

When using monofilament as your shooting line you should always "burn or melt" the cut end which will help keep the line from sliding through the crimp/sleeve if not crimped properly.

I have actually speared and landed blue water fish utilizing the burn or melting method WITHOUT actually crimping the crimp/sleeve at all. Oh bra, lucky uh!