Transcription can be repressed in a variety of ways, and therefore can be derepressed in different ways as well. A common mechanism is allosteric regulation. This is when a substrate binds a repressor protein and causes it to undergo a conformational change. If the repressor is bound upstream of a gene, such as in an operator sequence, then it would be repressing the gene's expression. This conformational change would take away the repressor’s ability to bind DNA, thus removing its repressive effect on transcription.
Another form of transcriptional derepression uses chromatin remodeling complexes. For transcription to occur, RNA polymerase needs to have access to the promoter sequence of the gene or it cannot bind the DNA. Sometimes these sequences are wrapped around nucleosomes or are in condensed heterochromatin regions, and are therefore inaccessible. Through different chromatin remodeling mechanisms these promoter sequences can become accessible to the RNA polymerase, and transcription becomes derepressed.
Transcriptional derepression may also occur at the level of transcription factor activation. Certain families of transcription factors are non-functional on their own because their active domains are blocked by another part of the protein. Substrate binding to this second, regulatory domain causes a conformational change in the protein to allows access to the active domain. This lets the transcription factor bind to DNA and serve its function, thus derepressing the transcription factor.