The Russians themselves were also detected to be building up troop numbers in Southern Kursk just opposite of Sumy. The Ukrainians cleverly used that to send troops to Sumy, which the Russians must have thought that it was a countermove to defend against them, to mask their true intentions of going on the offensive. The Ukrainians merely turned the operational table on to the Russians.

That being said, with hindsight, the earlier attacks on Belgorod by anti-Putin Russians could have been probing attacks to identify weakpoints in the Russian border. Or perhaps to distract the Russians and redirect troops away from other areas. Or both. All of these are clever and creative plans by Kyiv; and they played their hands well.