Question: For example:

[![screen capture](https://i.imgur.com/YnInEl3.gif)](https://i.imgur.com/YnInEl3.gif)

My explanation of what's happening in the gif: When a title has been hovered for a short amount of time, that title expands, pushing other content off the screen and grabbing the user's attention.

I've read that taking actions on hover is not desirable, rather only take action when the user takes explicit action (such as a click). Is that a principle and would it apply in this case?

Answer: 
Is hover the *only* way you can find out that information? Would someone on a tablet, or someone using keyboard without a mouse be able to find that full information without having to select that item fully? 

If hover is just one way to display it - an enhancement, so to speak - then it is OK. But if you *have* to use your mouse and hover over the item in order to find out the details, or you have to actually select and start watching the film before you get those details, then no, it isn't OK. 

Never penalise people who can't / don't use a mouse. Use hover for enhancements, but never for *the way*.
Dark patterns FTW?
==================

The problem with moving things around on hover is that the targets jump. This isn't really a problem for the item being expanded. As Jamezrp pointed out, that may be intentional. Maybe Netflix is using a dark pattern to try to increase engagement. I say *dark* because it's not an ideal interaction, but Netflix may have found the "negative" side effect to work in their or their users' favor.

Rule of thumb
=============

In general, you don't want to do this kind of expansion because it creates a certain amount of disorientation. If the thing being expanded (intentionally or not) is just one option among many in the user's consideration set, then they'll have to stumble a bit if they decide to try another option before committing to the click.

A better option
===============

If you're tempted to use a hover pattern (or it's tap equivalent in a touch UI) to provide expanded info, there is a better way. Rather than distorting the whole environment, just change the state of the item in question. In the Netflix example, the additional info could simply be layered over the cover image without any expansion. The cover attracted attention and now it becomes simply a backdrop.

So this ...

[![Standard cover](https://i.stack.imgur.com/0npk6.png)](https://i.stack.imgur.com/0npk6.png)

Becomes this ...

[![Expanded entry info without size change](https://i.stack.imgur.com/hJmmb.png)](https://i.stack.imgur.com/hJmmb.png)