A Precursor blocker is a Precursor artifact in The Precursor Legacy. It is a small cylinder made of Precursor metal. When active, a split halfway up shows a number of blue rings that emit a blue circular field and a humming noise. It can be deactivated by stepping on top of the device, in which its active state disappears; the shield goes down, the humming noise stops.
These devices are either surrounded by snow or ice, in which case the deflections cause you to slide around and become vulnerable to enemies or falling off cliffs. In order to be successful you must run towards the blocker and jump from the edge of the field landing on top of it. If you miss you will be propelled away.
They are found all around Snowy Mountain, with a few of them in front of the Lurker fortress. Jak had to de-activate several of these for a power cell in the "De-activate the Precursor blockers" mission.