Partial passwords are a terrible idea for a multitude of reasons but one in particular stands out to me. We have come to the general conclusion that humans shouldn't remember their passwords, they should remember one complicated pass key that is linked to a number of otherwise inaccessible secret keys used to interact with servers but, let's step back for a moment and assume people are remembering their passwords (which most people not on HN still do)...

A password has an entropy n, a higher n yields more security but comes at a cost in terms of memory power, I think it's reasonable to state that very high entropy passwords exhaust our memory quite quickly which is why password reuse is so common, this particular authentication is requiring the user to have a high entropy password n, and then is reducing it to an entropy m where m is at most as large as n (usually much smaller) and using that for authentication.

We, users and tech people, don't care about the cost to transmit bits over the wire or the cost to verify those bits on the far end (in this scenario) we care about the cost on the user's memory more than anything so why are we optimizing this puzzle to be least efficient for our most valuable resource.

When I first read this article I thought this practice was silly, now it strikes me as stupid... it is a transformation that simply discards some of the potential security of the system for no reason.

(Also, banks love ease of use, which is why up here in Canada they love to send out password dongles to customers that make it even easier to log in to a site than this while still requiring possession of a physical thing - and many of those dongles require additional authentication to activate whenever using, though not all)