This video is about an unusual volcano that straddles the border between China and North Korea, and has some really, really good animations drawn from tomography mapping of the plate dynamics around that area:

https://m.youtube.com/watch?v=3C2HVOB-g5s

Skip to the ten minute mark, but the whole darn thing was really well put together.

Basically, the plates can either be forced all the ay through to the core, or slip between the mantle and the plate above it.