The cap fits the ballpoint snugly and holds well.
It may be very tight, when you receive it, but after washing left nylon powder away and some use it will fit perfectly.
NOTE: The black Cap has a rougher surface than the other colors.
It plugs using a simple system, but great:
Press the silver plug through one or two layers of tissue packaging film into the corpus' opening. Tear or cut the rest of the film off. Now the plug fits very tight as the film adds pressure.