Why do you have to completely seal the black, when refilling Canon ink cartridges?

Canon iP4200 printer won't recognize ink levels after refilling cartridges?

• After refilling cartriges the printer doesn't recognize ink level and gives steady alarm. I know there is a way to override this alarm so that it will indicate ink tank levels and print but I don't know how. I do know how to make it just print with these tanks.

• Answer:

  In order to be able to use refilled cartridges, you have to disable the ink level indicator. You can't have it both ways. Do you want your printer to calculate how much ink you have left? Or do you want to use refilled cartridges? If you choose to use refilled cartridges, the override is easy. Install tank(s). Send print job to printer. You'll get a series of error messages (roughly 3 in total) about how dangerous it is to use refilled cartridges, and are you sure you really, really want to print. Answer yes/continue printing to all of these messages. Your last message should tell you to press and hold the resume button for 5 seconds, then release. Do it. You will have to repeat this for every refilled tank installed. Done!