How was Thanos able to defeat Hulk in Infinity War without using any stone?

Question

In the beginning of Avengers: Infinity War we saw that Hulk started beating up Thanos easily but suddenly Thanos overpowers him. Probably, the only way that would be possible, would be because Thanos used the power stone.

Whenever an infinity stone needs to be used, Thanos is supposed to make a fist with the Gauntlet and then a beam would come from the stone. However this time there was no beam, no finger flexing to make a fist, which means the power stone wasn't used to defeat Hulk.

So how come Hulk was defeated?

Answer 1

Thanos didn't use the power stone on the Hulk.

Thanos is a massively powerful being on his own, even without the stones. Note that the Power Stone wasn't glowing at the time and also Ebony Maw let Hulk and Thanos fight without any interference, saying:

No, let him have his fun...

To quote an official source:

Infinity War Writers Say Thanos Could've Beaten Hulk, Power Stone or Not, on Movieweb:

Avengers: Infinity War has brought up more than a few debates, but screenwriters Christopher Markus and Stephen McFeely have now revealed that Thanos would've beat the Hulk even if he didn't have the Infinity Gauntlet with the Power Stone, fueling more debate. The opening scene takes place on the Asgardian refugee ship after Thanos and his Black Order have taken it over to locate the Tesseract. In a last ditch effort to defeat Thanos, Loki sends the Hulk into a rage to fight. However, the fight doesn't go in the Hulk's favor.

Stephen McFeely believes that the Mad Titan would've destroyed the Hulk with his own bare hands in Infinity War. This would explain why Ebony Maw allows the fight to continue and makes the scene look like Thanos was just having some fun, like a cat playing with a mouse. Looking back at the scene, Maw and Thanos are pretty calm, even when it appears that the Hulk has the upper hand in the battle.

McFeely had this to say when asked if Thanos could still beat Hulk without help. I think he could. I may be speaking out of turn, but I think Thanos could kick the Hulk's ass without the (Power Stone).

The article further explains:

However, the opening Infinity War scene wasn't just to show that Thanos could beat the Hulk. Stephen McFeely and Christopher Markus wanted to establish the fact that Thanos is the biggest villain in the entire MCU. Marvel Studios boss Kevin Feige revealed early on that the first 5 minutes of Infinity War would show off the immense power of the Mad Titan and he was not kidding around.

McFeely explains, We want to announce Thanos as the biggest villain in the MCU, so he takes out the previous reigning champion. And by defeating Hulk relatively easily and savagely, hopefully there's a sense of dread over the course of the rest of the movie for anyone else who comes up against him.

You can read more in the interview with Christopher Markus and Stephen McFeely here.

Answer 2

Thanos is strong enough even without the Power stone or any other infinity stones. There are various evidences that suggests that,

Scene 1:

In the Avengers Infinity war movie, during the battle at Titan where Iron Man and Thanos fight one on one, Iron man uses his nonobots to make sure that Thanos cannot close his fist and activate the infinity stones. Then he launches a dozen of missiles at Thanos and Thanos is left unharmed after the explosion.

Scene 2:

In the Guardians of the Galaxy movie when Ronan decided to betray Thanos, Korath says this to him,

Master! You cannot! Thanos is the most powerful being in the universe.

Both these scenes should suggest that Thanos is powerful even without the stones. Given this with the reasons provided by Vishwa in his answer it is safe to conclude that Thanos is powerful enough to defeat Hulk.

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I strongly disagree with the Hulk being Loki theory here. Consider the folllowing points,

1. Here is what Russo brothers said about Banner unable to transform into Hulk,

So we thought an interesting direction to take him in is what if Banner, who typically uses the Hulk to solve crisis situations, what if the Hulk were no longer interested in solving those problems for Banner? So that the relationship is becoming increasingly dysfunctional. That’s what’s going on here. People have speculated whether there was some fear on the Hulk’s part about having to face Thanos again. But I think ultimately what it is, is that he’s tired of playing hero to Bruce Banner.

2. Banner tries to summon Hulk two times in the movies where no one is watching, one time when he was inside the Hulk buster suite. If he was Loki why would he do that?
3. Banner when speaking with Tony has knowledge about events that occurred during Age of Ultron when he says,

Tony, you lost another super-bot?

and

This suit's already kicked the crap outta the Hulk.

He knows how to use the Hulk buster suite which he helped to design.

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References:

1. Guardians of the Galaxy - Transcript
2. Russo brothers about Hulk