Subspace variance
A subspace variance was radiated from cloaked ships at warp speeds, according to T'Rul. This variance was apparently detectable by the Jem'Hadar.

When the cloaked  was detected by the Jem'Hadar in the Gamma Quadrant in 2371, Benjamin Sisko's first instinct was to decloak and prepare to fight them. T'Rul stated that dropping out of warp would eliminate the subspace variance. The Defiant would then become undetectable. ()

de:Subraumvarianz
variance