Their 'Master' sought to acquire the six Infinity Stones.
Thus far, he'd managed one.
And he'd lost it to Loki's keeping.
But its theft was not without price. Loki knew the workings of the Mind Gem. Knew its use to burrow deep within a man's heart and exploit all traits there residing. The loyal, to thoughtless obedience. The kind, to witless generosity. The vengeful, to madness. In a mortal, the effects were complete and overpowering, as though all the rest of them were driven aside from fear of this one, cardinal trait. Merely residing near the gem was ill-advised. Its powers were fierce. They sucked at the mind like a maelstrom to the soul's raw core.
In one of the older races, it was different. An elf. Dwarf. Giant. God. Their minds were less easily overcome. Perhaps something the mortals would adapt to. But they hadn't yet. Loki counted on that.
Counted on it more than he'd planned.
He held the Mind Stone at the price of the full faculty of his own mind. Placing the spear crafted to house and focus the gem into his hand, Loki felt the mad titan dig his claws in. It was painless, and stunning, all at once, like an unexpected drop from some height.
He'd rather hoped the titan would neglect that particular precaution.
He gasped for his breath.
The 'Other', his latest keeper, leered at him.
He'd been told, then, that the procedure was taken that he might communicate with the 'Other' at any time. It was a thing meant to both channel communication between them and to track the feet of its bearer. "That your army may find you."
The creature leaned forward.
"Should such things be truly necessary," it hissed.
But Loki knew more than that. The Tesseract had shown him more than these creatures knew. Thus used, the gem would allow the 'Other' audience of his every word.
At any time, Thanos could press just harder and force his will to break. Force him to do as he would not, and he would have no knowledge of it until the thing was done. As with any mortal. Thanos had the strength.
The mad titan knew his mind. Knew his weakness.
The weakness of the one who had fallen.
A weakness that might yet be exploited, and might not be necessary. Thanos might possess enough power, even having the Gem far from his grip, to force his hand.
It made everything harder. He'd be forced to veil his hand more fully than he'd counted on.
He'd won at impossible odds before.
And what had he left to lose.
He would yet out-smart the Mad Titan.
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I don't remember where I heard about how the stone worked. It was not something I made up myself. That came from somewhere else (someone was comparing it to the serum used on Captain America and saying how similar they actually were).
The Other-can-hear-everything-Loki-says part came from the movie, actually. Loki just SAYS 'send the rest.' He doesn't DO anything – not even any touching-my-temple-bs like Professor X – he just says it.
Now, granted, Thor also just talks and the rest of the team responds. But. I like this theory. Take it or leave it.