The basic idea is open addressing, but without any redirections (thus making swiss tables very memory efficient). Also, swiss tables store a densely packed array of metadata, sort of like UNIX file system inodes and bit maps (see OSTEP: Chapter 40 File System Implementation). Thus, we can use the metadata to tell if an entry in the table is present or not (see the details at https://abseil.io/about/design/swisstables).
And there’s an excellent video about it on youtube.
Finally, let’s see the performance differences on the counting words benchmark.