In the 2 masculine plural positions, the subject Waaw is dropped before Heavy Noon.
The preceding letter carries Dhammah as an indicator of the deleted Waaw.
Example:
يَفْعَلُونَ
becomes
لَيَفْعَلُنَّ
Recognition clue: Dhammah before Heavy Noon points to these masculine plural positions.
Opening interactive controls...
Two Masculine Plural Forms: Waaw Drops
Structural Rules of Heavy and Light Noon-e-Ta'keed
In the 2 masculine plural positions, the subject Waaw is dropped before Heavy Noon.
The preceding letter carries Dhammah as an indicator of the deleted Waaw.
Example:
يَفْعَلُونَ
becomes
لَيَفْعَلُنَّ
Recognition clue: Dhammah before Heavy Noon points to these masculine plural positions.