It's already well explained in a sibling comment, but on a more conceptual basis, while commits are interpreted as diffs on the fly, a commit is a single (immutable) snapshot. So in these terms, "splitting a commit" amounts to introducing an intermediate snapshot. Having that in mind, it should become clear, that using Git you create the snapshot by working from the previous or next commit (what ever suits you more), bringing it to the state, you like it to be and commit. (In theory you could create that intermediate snapshot from any commit, but likely you want to do it from on of the direct neighbors.)