An interval time trial is a time trial in which consists of interval and rest periods. However, your bike would only move forward during the interval periods. The idea is to combine the excitement of racing with the benefits you get from doing a structured workout.
The interval and rest periods would be defined on time, and your placement in the results would be based on how far you travel. The types of intervals done would be configurable similar to a workout. For instance, you could have a time trial with 8 sets of 2 minute intervals and 2 minute rest periods. Another time trial could consist of a pyramid of 1,2,3,4,5 minute intervals with similar rest periods.
So why add this when Zwift already has time trials and group workouts? Time trials are fun, competitive, and really push you, but you don’t get the same benefits of a structured workout. The group workouts provide a good structured workout, but are kind of boring, and the banding is artificial. By combining these two features, you would get a fun, competitive structured workout. Zwift could further enhance this, by letting you compare your results to previous workouts. Also, like a workout, they could be done on your own, or as part of a group, just not a banded group.
The interval time trials will not replace group workouts or time trials. There are many types of workouts that would wouldn’t make sense such as over/under intervals, or even moderate intensity workouts. Time trials and races would still exist. This would add a way to get a structured workout in a fun, competitive way that could be compared against previous results.