For example, changes from one frame to the next are encoded in rectangular areas called "superblocks" (similar to a https://en.wikipedia.org/wiki/Macroblock). You can "move" the blocks (warp them), define their change in terms of other parts of the same frame (intra-frame prediction) or by referencing previous frames (inter-frame prediction), and so on... but you have to do it within a block, as that's the basic element of the encoding.
The more tightly you can define blocks around the areas that are actually changing from frame to frame, the better. Also, it takes data to describe where these blocks are, so there are special limitations on how blocks are defined, to minimise how many bits are needed to describe them.
AV2 now lets you define blocks differently, which makes it easier to fit them around the areas of the frame that are changing. It has also doubled the size of the largest block, so if you have some really big movement on screen, it takes fewer blocks to encode that.
That's just one change, the headline improvement comes from all the different changes, but this is an important one.
There is new cleverness in the encoders, but they need to be given the tools to express that cleverness -- new agreement about what types of transforms, predictions, etc. are allowed and can be encoded in the bitstream.