For anyone that was confused about what chording meant like I was:
1. Mark a tile that definitely has a bomb with a flag by right-clicking
2. Double-click left mouse on a number next to this flag that is satisfied to clear all dirt around it; your character should do a spin if done correctly
For example, if you mark a bomb with a flag and that marked bomb has a "1" next to it, you can double-click the "1" itself and it'll clear all other dirt around the "1" since you know for sure there aren't any other bombs touching it. You can do this with any number so long as it already has the correct amount of flagged bombs touching it.