How the 90-days rule compliance is controlled?

Actually, the 90-days rule is enforced in Russian visa registration procedure only. Immigration officers at the Russian border control still do not count the real length of your stay in Russia. Thus some foreign nationals having Russian business visas and permanently living in Russia prefer to cross the Russian border with neighboring countries (Ukraine, Belarus, Latvia, Estonia, Lithuania) and re-enter in a one or a few days. This allows them to re-new Russian migration card and to have proper Russian visa registration in every day of staying in Russia.