If you remember before 5 years ago all urls was unencrypted in SERP and lot of tools using this for capturing "keywords" and linking them to pages. After they introduce this in 2010 they begin rollout in few years and today only way to see keywords is in SearchConsole. Of course encryption is for "to improve your search quality and to provide better service". Original text can be seen here. Please note "provide better service" there. This is tricky!
So imagine that you search for moz and here is actual URL i can see now:
https://www.google.bg/search?q=moz&ie=utf-8&oe=utf-8&gws_rd=cr&ei=4wNVVpnZBYGoUZiXh4AG
you can definitely see keyword there in ?q=moz now first result is Moz.com and it's URL is:
https://www.google.bg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjenMnUq6rJAhXIRBQKHXuVCcUQFggfMAA&url=https%3A%2F%2Fmoz.com%2F&usg=AFQjCNHNW83KUfvLcZOMILlYW49NobxUig&sig2=nOVvQ05KIPrGB3XFAFmIGgAs you can clearly see - there isn't keyword anymore but everything comes with encrypted data (ved, usg, sig2). This link /url is actual redirector that count your click on specific result and position.
Now if i click on 1st result and go in Moz.com i can scroll down and i find "this isn't MOZ i'm looking for" so within some time (few seconds) i will return to SERP. This is actual "dwell time" and bounce back to SERP. It's negative signal because it's show to Google that result he return for first place isn't correct with human verification. Now back on same SERP i can see Moz in Wikipedia:
https://www.google.bg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=19&cad=rja&uact=8&ved=0ahUKEwjenMnUq6rJAhXIRBQKHXuVCcUQFghhMBI&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FMoz_(marketing_software)&usg=AFQjCNGCgqmsKNIdaZdGrbugf8bJk6NhTg&sig2=jS-vt68NFtD5YhgSV4lTGwIf i click this and i doesn't have return to SERP anymore this give to Google enough to calculate bounce rate for this site (only to return in SERP) so give Wikipedia some "goal completition". And time for next search can be used to calculated "time on site".
And since all searches are encrypted they knows when specific user search for something and when they make new search based on already returned data. Example is "Napoleon". This can be anything - french emperor, movie, cake, drink and other things. So now i can do subsequent search "Napoleon height". This is example how one search can give me enough information to do another refined search. Other good example can be "32 us president". Then i can type "franklin d roosevelt height".
This was explained much better in closing MozCon 2015 presentation "SEO in a Two Algorithm World ":
http://www.slideshare.net/randfish/onsite-seo-in-2015-an-elegant-weapon-for-a-more-civilized-marketer
and you should see it. There also shown few tests inside with terrific results.