I think "contiguous" does a great job of making "it shrinks all squares below it and in the same row segment" more concise and understandable, even if just for the subset of people who understand what contiguous means. That being said, I wouldn't underestimate the intelligence of the average english speaker that much. Contiguous is a pretty common word.

But a good compromise would be a video or something demonstrating exactly what you mean.

I agree describing it in text might be a problem. I would generate a few pictures showing different ways the "blast lines" interact with the board. I wouldn't go through all the more interesting chain reactions (part of the fun is finding them). But I think it would make the game easier to understand for a first time player.