DNA topoisomerases such as bacterial topoisomerase II (gyrase) convert relaxed circular DNA into supercoiled DNA.  The DNA Topoisomerase II (Gyrase) Assay Kit is based on the principle that the supercoiled DNA and relaxed DNA yield different fluorescent intensity when interact with fluorescence dye H19.  The relaxed DNA suppresses the fluorescent intensity much more than the supercoiled DNA.  Therefore, when the relaxed DNA is converted into its supercoiled form, the fluorescent signal increases.  The change of fluorescence intensity is used to measure the supercoiling reaction.