From Longman Dictionary of Contemporary Englishtighten up phrasal verbif a team or group tightens up, they start working together more effectivelytighten something ↔ up We have tightened up the defence and are winning matches as a result. → tighten→ See Verb table
Examples from the Corpus
tighten up• The music industry is determined to tighten up on the illegal copying of CDs.• The prime minister has promised to tighten up the law on carbon dioxide emissions. 