Conventional bounce rate in most cases is defined as rate of visitors only visiting one page.

But with this definition "a bounce" does not necessarily mean a visitor is not interested in the page content he saw.

A discussion on this could be found e.g. in #9199 starting #9199 (comment)

Literature argues to same direction telling a bounce can also be defined as a very short visit (5-10s) with no other interaction,
e.g. Web Analytics: An Hour a Day. Avinsh Kaushik, 2007, p. 168 - 170, Sybex, ISBN 0-470-13065-2, mentioned in wikipedia (in german part: https://de.wikipedia.org/wiki/Bounce_Rate)

In the past it was from technical pov not possible to detected and with this classify the length of only onepage visits
(maybe this was also an influencing factor for conventional bounce definition?)
With great Piwik's new heart beat feature it is possible classify the length of only onepage visits.

=> with all this background and possibilities given,
it's seems to be the obvious consequence to
to start giving another new characteristic ratio to describe performance of a webpage:
"the rate of visitors interested in a page"

This one could be possibly determined by
the rate of visitors staying longer than e.g. 10 seconds and /or do interaction (e.g. event) with one of the elements on the websites.

This is the advanced version of conventional bounce rate
only possible with piwik :-)
USP