Once they establish that connection, once their souls are in synchrony, their minds can interface with even higher bandwidth. Yvette natively has that much bandwidth with Nai because they're sharing a gate, and that's how she can feel this love no matter what, that's why Nai thought that of course she was going to be feel snuggled. She's sharing in this bond with them.
But it's not just the feeling. There is a purpose to what they're doing. With their gates connected to each other like this, the twins can get to work. They look, and reach towards their sister, and go all the way into her heart and mind and body, all the way to the other dimension.
And they assess.
Where's the damage? What's going wrong? What's missing, what's hurting, what's leaking? Where are you in pain? How can they help? It's like surgery, they have to use their minds as their scalpel and to carefully and intimately know all of the ways in which the humans have hurt her. Pay close attention to their patient, comfort her, and heal her.
So it's no wonder Zash gets so exhausted, so physically and emotionally wrung out. He's not angry, never angry, but he's so, so sad and hurting for his sister, for all the pain and damage she's had to gone through. He knows exactly how much she's clamped down on her hurt and discomfort so that she could keep helping the humans, how much she's had to ignore her needs and her wishes because there was something more important on the line.