Harris was clocked at seventy-something in a 55 MPH zone, and the cop who caught him tried to make a traffic stop. Instead of pulling over, Harris sped away. And so a massive pursuit with lots of cops began. Apparently, Harris' driving (as recorded on dashcam) was pretty reckless. About 6 minutes into the pursuit, Scott (a cop) rammed Harris' car in an attempt to end the pursuit. Harris lost control, crashed, and wound up a quadriplegic. He sued Scott for excessive force, and Scott moved to dismiss the case under qualified immunity. The district court (and later the appellate court) found that there were sufficient factual disputes to require that the case be decided by a jury, and this case made its way to the Supreme Court.
Harris' argument went like this: he was a very safe driver whilst fleeing from the police, and didn't endanger anyone, and so the requirements for the use of deadly force established in Tennessee v. Garner were not met. Scott's action of ramming the car posed a substantial risk of killing or seriously injuring him (as evidenced by Harris' paralysis), and therefore it was deadly force. Therefore, it was unreasonable.
The Supreme Court made a few interesting points in this decision. It held that Garner didn't actually create rigid rules about the circumstances under which deadly force would be permitted, it merely applied the reasonableness standard to a particular kind of force in a particular situation. That's interesting because it means that for constitutional purposes, it's not really important to figure out whether or not Scott's actions constituted deadly force; it's only important to figure out whether or not they were reasonable. More on that in a moment. The court also ruled that although legitimate disputes of fact would have had to be decided by a jury, in this case there was no legitimate factual dispute because the dashcam footage made it obvious that Harris' version of events was bullshit (I think the court said fiction, but I'm pretty sure they meant bullshit). The Supreme Court held that no jury could have reasonably believed Harris over the videotape, so the lower courts should have made the qualified immunity decision based on the facts as shown by the camera.
And here we get to the reasonableness decision. The court observed that Harris' flight from the police put the lives of the police and of innocent bystanders at risk, but that ramming Harris' car put Harris at even greater risk. Still, considering that the whole situation was Harris' fault to begin with, the court found that to be reasonable. The court specifically declined to create a rule requiring officers to break off a pursuit, since doing so doesn't guarantee that the suspect will stop driving recklessly, and since the court didn't want to create an incentive for suspects to run from the cops. Since Scott's actions were reasonable, he didn't violate Harris' constitutional rights and he was granted qualified immunity.
I remember when I first heard about this case, I thought that it was basically making a rule that if someone is fleeing from the cops then the liability for whatever happens falls on their shoulders. But I was wrong; it's not that simple. This decision doesn't prevent the police from incurring liability for their actions. Like Garner, it just applies the reasonableness standard to a particular kind of force in a particular situation.