How does Terrorblade's Sunder ability work?

How does exactly Terrorblade's Sunder ability works?

To be precise I would like to know what will happen if Terrorblade has X% of its health and enemy hero has Y% and Terrorblade casts his ability on the enemy? For all possible X and Y values.

The reason I am asking is that I do not quite understand the limitations of the ability.

It is said in gamepedia that:

Some hit points must remain.
Minimum Health: 25/20/15%

Does it mean that after casting the ability on other hero the hero will have at least 25/20/15% of health left? What will be health of Terrorblade in this case?
Or does it mean that you can not cast the ability when Terrorblade has less than 25/20/15% health? Or something else?

dota-2

Pictures about "How does Terrorblade's Sunder ability work?"

How does Terrorblade's Sunder ability work? - High-rise Buildings

How does Terrorblade's Sunder ability work? - Women at the Meeting

How does Terrorblade's Sunder ability work? - Woman Working in Home Office

Does Sunder work against ice blast?

Sunder allows Terrorblade (or an ally) to regain HP while affected by Ice Blast.

How do you play Terrorblade effectively?

Terrorblade is the demon marauder--an outlaw hellion whom even other demons fear. A cosmic iconoclast, he stole from the Demon Lords, ignored the codified rites that should have bound his behavior, and broke every law of the seven Infernal Regions.

Is Terrorblade a demon?

Lina

Laguna Blade is one of the best tools to kill Terrorblade before he can use Sunder due to its high magical (pure with. Aghanim's Scepter) burst damage.

Dragon Slave and Light Strike Array allows Lina to clear Terrorblade's illusions from both, Conjure Image and. Manta Style.

Why TERRORBLADE is THE BEST 1v9 HERO - FREE MMR HYPERCARRY - Dota 2 Guide

Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: ThisIsEngineering, SevenStorm JUHASZIMRUS, RF._.studio, Ivan Samkov