The iStake, named as such by Molly, is strapped on the person's torso with a wooden tip aimed for the vampire's heart that will kill them faster than a bullet if triggered. It can be triggered if tampered with or by an application via Molly's iPhone, hence its name. The iStake is resistant to all kinds of damage.
So far, the iStake device has been used on only a few vampires. The first two we saw were Eric Northman and Bill Compton, who were both equipped with one on their mission to find Russell Edgington. After the duo located Russell, Molly removed their iStakes (since finding Russell equaled the Authority pardoning Eric and Bill). Subsequently Molly put an iStake on Russell since Roman didn't want to get his hands dirty executing him. The stake was tampered with, however, by Salome, who was turning the tables on Roman and allowing Russell to kill Roman instead. Finally, after Molly defected from the Authority in an attempt to help Eric Northman escape, she was sentenced to a very ironic true death utilizing her own device. In what was so far the only time we saw the iStake in action, she met the true death through it in front of the new Authority.