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Submounts are triggered via the follow_link inode operation, so in some ways these are treated like symlinks...
The short answer is that the mount will be triggered whenever you walk a path in such a way that, if this component were a symlink it would be resolved to its target.
Longer answer:
If the place where you transition into a new filesystem is in the middle of a path, then generally the path will be resolved. If it's the last component of the path, then it depends on whether the LOOKUP_FOLLOW link flag is set in nameidata in the kernel. That varies with the type of operation -- for instance, lstat() won't have that set, but a "normal" stat() generally will.