@omnimon11  
The artist himself decided to leave it up to interpretation.

So, no-one here has an exact idea of what it is, exactly, Sans is doing to Twilight?

@Project Z  
Oh definitely. One of the ideas I’ve been thinking of is if Frisk was given a pony’s soul to help defend Equestria.

@Red-Supernova  
I would say a ponies soul has a fair amount of determination. For instance Twilight fighting tirek or Pinkie Pie trying to become cranky doodles friend are just a few examples of many in mlp where determination has been shown.

@Ereiam  
Yes, you are right! But does a pony’s soul possess Determination?
 
Oh damn, the results for that would be perfect for a crossover.

@Red-Supernova  
And yet, ponies seem much more capable of love, hope and compassion than humans (granted, the show also proved that there are exceptions to that, so maybe you’re onto something here).
 
I also wonder what would happen if a monster tried to absorb a pony’s soul.

@Ereiam  
I’ve been thinking about that for awhile. My guess is that a pony’s soul is slightly stronger or as equal as a human’s soul.

@Background Pony #74BD  
Yes that and his powers are based on justice. He could not harm the player with his gaster blasters. Not sure about the bones though.

@Keith Mowz  
Which raises the question of how a pony soul compares to human and monster souls.

@Background Pony #74BD  
While true, that was also because the player was a human.

@Project Z  
The only thing prevented him from killing innocent player on sight is promise to Toriel. Do the math.

@Project Z  
That’s true. Twilight has very good judgement.  
I think she is never afraid to fight against those whom wish to harm her or her friends but she would never use her magic to kill her enemies.

@Red-Supernova  
Based on how Twilight wouldn’t ever hurt anypony or monster Sans attacks couldn’t hurt her because he states in the game that his attacks are justice based meaning the more evil the person/pony is the more damage he does. If he tried attacking the player on a pacifist run his attacks wouldn’t do a thing.
 
Besides he judges the player no matter what whether it be the pacifist route or genocide route or neutral route. So I’m sure our Princess of Friendship did no harm and stayed true to Celestias teachings in the end, sans just wanted to make sure. :)

@Keith Mowz  
Oh yeah….I forgot about them.
 
@Background Pony #74BD  
I’m not sure but I bet it would be an interesting topic to interpret in MLP & Undertale crossover fics. And a Rainbow Power-ifed Sans would be funny.  
I bet it would shatter a post-Genocide Run Frisk.

Here comes the toss! And she’s out of here!

@Red-Supernova  
I wonder what would happen if he received Elements of Harmony attack, or Rainbow Power laser. He’s not really a “hero”, and not exactly sinless. Would EoH/RP make him experience all physical and mental pain he inflicted on others all at once? Or simply banish him to void?

@Red-Supernova  
[You feel the pinkie clones crawling up your back.]

@Project Z  
I just hope Sans isn’t doing anything bad to Twilight. She would never kill anyone.

“Are you ready to have a bad time?. Twilight.”

Perhaps this is his way of judging her actions? Not a very subtle judging system but with the outfit he wears subtle just isn’t the first thing to come to mind.

@Background Pony #74BD  
He’s turning her soul blue, I presume.

Sans is new Sombra?