The Manheim Effect was the term used by Lieutenant Commander Data to describe the temporal distortions created as a byproduct of Doctor Paul Manheim's time/gravity experiments. The Effect, which was caused by Manheim opening a "window" into another dimension on Vandor IV, consisted of a specific moment of time which repeated itself.
The effect was first felt in 2364 by the crew of the USS Enterprise, which was in close proximity to the Vandor system. Investigating, the crew experienced the effect again, and learned that it was also experienced on the USS Lalo, a colony on Coltar IV, and in the Ilecom system, thousands of light years away. Questioning Dr. Manheim, they learned that the effect would continue unless the window into another dimension were to be closed.
As Commander Data seemed to have an easier time withstanding the effects of the time distortions, Captain Picard ordered him to beam down to Manheim's laboratory alone to seal the breach. As Data prepared to close the window by adding a specific amount of antimatter, he experienced another distortion, wherein three versions of Data from different points along the time continuum appeared. Realizing that only one of them was in the correct time period for adding the antimatter, they merged as the antimatter was added at the correct time and the distortions ceased. Responding to Captain Picard, who asked if the breach was closed, Data stated that it was "well patched." (TNG: "We'll Always Have Paris")