An inverse tachyon pulse is an unorthodox method of scanning. It is used within subspace or dimensional anomalies where conventional sensors or scanners cannot penetrate the subspace barrier. (TNG: "All Good Things...")
The use of a tachyon pulse in this way was first conceived by Data aboard the USS Pasteur in an unknown, alternate future timeframe. Creating the pulse required the modification of the vessel's deflector. The pulse was used to scan the Devron system for temporal disturbances, in order to locate an anti-time eruption. To make the modifications and to scan the local area required approximately fourteen hours.
The same method was also used by the android aboard the USS Enterprise-D in an alternate 2370 to scan the interior of the anomaly - now in existence - so that a clear picture of the interior could be obtained. The idea was suggested to him by Captain Jean-Luc Picard, who had been present in the future timeframe when the original pulse was suggested. It was also used by the Enterprise-D of an alternate 2364 for the same purpose.
As it turned out, the tachyon pulse was unable to fully penetrate the anomaly, and the present Enterprise used a tomographic imaging scanner to scan the interior of the anomaly. The tomographic scan showed three tachyon pulses converging at the center of the anomaly, meaning the convergence of the three pulses from the three time periods (in the past and present, the Enterprise-D, and in the future the Pasteur) was what originally caused the eruption.
