To understand what Crossfire Hurricane was all about, it helps to begin with the realization that it was not the FBI's first choice--in the big picture it amounts to a fall back option that was more or less forced on the FBI by circumstances beyond their control.
Make no mistake about it--the FBI was all in with the effort to enable a Democrat presidential handoff from Obama to Clinton. In modern political campaigns, intelligence and data in digital form is essential to success, and that's where the FBI came in. Running informants, such as Halper, against the GOP wasn't going to guarantee a Clinton victory--not in and of itself. However, access to bulk amounts of sensitive inside data could play a significant role. And the FBI had that access, in the form of access to raw NSA data, which means just about all digital communications in the world. Remember Nellie Ohr? The former CIA contractor and former employee of Fusion GPS? She became a ham radio operator during the 2016 campaign, so I guess she understood what it takes to fly under the Deep State radar.
Now, the FBI wasn't about to do something totally stupid, like mine the NSA for data and ship boatloads of it over to the DNC or some other intermediary to the Clinton campaign. Not as an institution. No, the smart way to do this would be to hire non-government contractors--we'll call them Fusion GPS and Crowdstrike--to provide the FBI with "analytical assistance." As if the FBI didn't have an army of analysts already. And then give these contractors total access to NSA data without telling NSA, who would have thought that only FBI employees were combing through their data. You can read the gory details in Jeff Carlson's excellent article: The FBI’s Outside Contractors, DNC Servers & Crowdstrike.
The beauty of this approach was that it totally bypassed legal controls. There was no need to falsify things in writing, no need to make stuff up to open a Full (Counterintelligence) Investigation (FI) on a US Person (USPER), and then lie to the FISC to get FISA coverage on the USPER. No need to have to regularly renew the FISA and lie all over again to the FISC each time. Of course, it wasn't as if the FISC was doing much besides rubber stamping FISA applications but, hey, who needs the bother? And besides, better safe than sorry. Because, in the unlikely event that the FBI would be questioned about this arrangement, they could just play dumb: Gosh, we didn't know the contractors were looking at all that stuff! How do you prove criminal intent?
So this cute arrangement was humming along, working like a charm. For how long? That's hard to say, but we know that it was in operation no later than December of 2015. And the way that came to light was that in the Spring of 2016 the unlikely event actually occurred: Admiral Mike Rogers, head of NSA, learned of irregularities in FBI accessing of NSA data and did an audit of the activity. That audit covered the period beginning from December 2015, and it discovered that fully 85% of the queries failed to comply with what are called "minimization procedures" (procedures designed to shield the identities of persons who fell within certain criteria--like, USPERs who had no connection to intelligence or terrorist activities). Then, as if Rogers learning about this weren't bad enough, Rogers went and blew a whistle--to the FISC itself--and put a stop to it all. That was in April, 2016, just as it was becoming obvious that Trump was going to be the GOP candidate for President.