You can split intervals when you create a workout. This is useful, especially when creating workouts based on playlists where you want the intervals to match the music exactly.
Here is a video that will show you how to split intervals
The position of the "split" is determined by the location of the time-indicator-bar. When you open up the Session Designer it will be placed at the 0:00 mark on your timeline visualized by a grey circle.
When you have added some intervals you can move the time-indicator-bar by either dragging the bar with the mouse or by clicking somewhere on the timeline below your workout. You can also input a specific location in the timer at the top left side of the display and the cursor will move to that exact position.
When you have placed the time-indicator-bar at the right location you click "split interval" in the top menu and the interval will split at the given time.
You can also click the play button and simply split the intervals while the time-indicator-bar is moving across the workout. If you choose this method, it is helpful to zoom in first to ensure you split the interval exactly where you want to.