Complex sentence: When the preceding syllable is high but the following is toneless, then the medial toneless syllable adopts a high-tone onset from the preceding syllable, resulting in a phonetic falling (high--low) tone.
Split into simpler sentences: When the preceding syllable is high but the following is tuneless, the medial toneless syllable adopts a high-tone onset from the preceding syllable. That results in a phonetic falling (high--low) tone.