Complex sentence: Debian testing is a cyclical development branch and before each release of Debian stable, Debian testing is frozen (and thus not rolling) affecting rolling distributions based on it--like LMDE and antiX.
Split into simpler sentences: Debian testing is a cyclical development branch and is thus frozen before each release of Debian stable. During this time, Debian testing is no longer rolling, which affects rolling distributions based on it--like LMDE and antiX.