In plain English: the workout you merge, starts at the end of the ongoing interval.
I’ve been using this a lot for several reasons. I start with a standard warm-up, which is a workout on it’s own. In the last step of the warmup, I decide what I will do. Usually I do what is planned, but if the warmup feels like calling for something else, I have the opportunity to deviate from the plan without the need to restart the whole thing. I have blocks like LIT, Over-Under, Sweetspot and append those after the warmup. Another advantage of these blocks is that it is easy to extend for example a LIT block with another LIT block. My LIT block is 30min with regular small changes in target power to avoid the boring super steady blocks. I’ll do 2 to 4 of those and then add the Cooldown block at any moment if I decide that it’s time to stop.
Workout design on the fly, superb functionality!