Anyone know how to create this video effect?

If you look frame by frame, the video hold a frame just slightly before the "effect" appeared. (in Premiere pro there is an option called hold frame ---> right click on a video clip ---> hold frame)
the effect fades in slowly (cross dissolve).
You should have a liquid video as video #2, but the video should start from small amount to big (and put it above video #1 in the timeline), there are a lot of liquid footages or other.
Play around with the Video #2 opacity blend mode (darken, multiply, color burn etc...) and see what work best for you.
Incase the colors are too harsh for your liking, you can also increase/reduce the opacity level to reduce the effect intensity.
this is an easier way to achieve that.
there is also a gaussian blur mask on the background once video #2 gets bigger.

your clip reminded me a more advanced technique used in US trailer.
An effect they used for the title cards:
trailer: https://www.youtube.com/watch?v=hNCmb-4oXJA
learn:https://www.youtube.com/watch?v=wHmFpSi2KBI

But in your video I think they used an easier method (something similar to what I've described), my english isn't great so apologies for that.

 

Thanks a lot

 

you are more than welcome mate, if you have any questions or I wasn't clear enough let me know.
Also, if you need further help don't hesitate to send me a PM.

 

Hi @animelrhl
I have a feeling that you can do it using Displacement Maps and an ink transition...

Look the "error" at 08:04: https://www.youtube.com/watch?v=JzJohgzY4S0