Not entirely it also uses bits of color ink to make the black and white page look better, at least according to the printer companies who conveniently also benefit by selling you more ink.

How can it make black look better by adding yellow to it?

Black ink isn't perfectly black. Adding other colors of ink makes it darker. If you don't really care about how dark the result is, the term you're probably looking for in settings is "rich black."

There is a few bits you can flip on the Firmware side to fix that.

Woah! I never considered that until now. I'll bet you're right.

> But then, how do B&W laser printer allow for tracing ?

They don't, because they don't need to. The point of the yellow dots is to prevent the printing of counterfeit currency, which itself tends to require the use of more ink colors than only black. It's probably possible to refill a "black" ink/toner cartridge with the exact shade of green (or whatever) to replicate the color of a currency note, but if it was easy to do then there'd probably be a lot more counterfeit bills floating around.

The ink/toner is also half the battle. The other half is the paper, since obviously the US Treasury doesn't use ordinary printer paper to print $20 bills. The usual trick is to take a $1 or $5 bill, bleach it (or otherwise remove the existing printing), and print a $20 bill design onto it - but that's easier said than done, due to both the ink/toner color issue mentioned above and due to the difficulty of getting the donor bill exactly aligned (and doing so again, in the exact same way, for the other side of the bill).