From the level-by-level mechanics of Ebb and Flow:
Level 7
Thrawn has +50% Counter Chance, +50% Tenacity, and -50% Speed while any enemies are Fractured. Whenever another Empire ally uses a Special ability while Thrawn is active, that ally gains 15% Turn Meter and, if any enemies are Fractured, Thrawn loses 15% Turn Meter.
Level 8
Thrawn has +100% Counter Chance, +100% Tenacity, and -50% Speed while any enemies are Fractured. Whenever another Empire ally uses a Special ability while Thrawn is active, that ally gains 15% Turn Meter and, if any enemies are Fractured, Thrawn and Fractured enemies lose 15% Turn Meter.
What you're gaining with the Zeta is:
+ 50% more counter chance (spreads Ability Block AND Stun, works great if Vader is on your team)
+ 50% more tenacity
+ The fractured enemy also loses turn meter when an Empire ally uses a special ability.
Thrawn already loses TM for each special before the Zeta, which is what keeps them fractured longer.
The Zeta just means that the fractured enemy will have longer to wait to fill their TM and take a turn, once fracture wears off by either Thrawn dying or Thrawn taking a turn.