I disagree with the conclusion in Ryan Thompson's answer, though the following interpretation is mechanically the same. I just came in here to clear some things up and show an alternate conclusion that can be made. That is, this spell does in fact turn thoughts into a physical object as a permanent transformation, and does not copy them.
This actually relies on the fact that the spell is worded as to say that if you're reading or manipulating someone's memories with a spell or ability and you're concentrating on that, you can use this cantrip to transform the thoughts or memories you read, rather than your own, into a thought strand.
This means in this context that the definition of pull actually means to magically pull something out of the mind and turn it into something physical, in this case a thought strand.
Here's where it gets weird. Even if you're reading or altering someone's mind/memories, it wouldn't make sense that it's actually getting pulled from their mind. In essence, since the range of the spell is self, you're actually effectively turning yourself into a ticker tape machine, meaning that you're actually pulling this information from your own mind as you read/alter someone's mind/memories. What would effectively be something you remember sensing (i.e. through detect thoughts or modify memory).
Like Ryan Thompson said, being able to immediately undo the removal of the thought by reading it back into your mind makes it odd, but it seems that's actually an intended potential interaction. See, these memory strands are very valuable to Dimir operatives, and are in fact traded as a form of currency. It wouldn't make sense that trading information as a currency would require losing access to that information forever, as this would not benefit a Dimir agent.
In fact, this benefits a Dimir agent in that they're able to effectively remove incriminating memories from their mind if it works as described.
This cantrip, thus is similar to the process of putting ones thoughts to paper, though in a more esoteric fashion that can only be accessed through being able to cast this cantrip or detect thoughts. They're basically temporary books that (based on the spell duration) last for 8 hours, except they can contain information of any level of granularity so long as it meets the requirement of being a single "memory", an "idea", or a "message".
Then again, this is just based off of experience with both using this spell as a player and having to moderate usage of this spell as a GM, so the information above might not be correct. It certainly seems to work best in this manner, however.