Splitting in Smooth Gestures means taking the current tab and those to the right and moving them to a separate window. Merging takes the current window's tabs and appends them to the last focused window.
One annoying thing it does on OS X: I have to right click twice to get a right click dialog. Also, I had to delete some shortcuts that overwrote my text navigation shortcuts (like skip by word with alt + left/right arrow).
Update: "Smooth Gestures" are better than "Mouse Stroke", thanks a lot!
PS: You can not possibly make an "productivity addons" collection without Mouse gestures, they increase your browsing speed easily by 100%.