The key to the hammer intervals is that the fast sections are run at your long interval pace and the hammer intervals are a sprint. So it helps if you have an idea of your paces for different workouts. But the difference is about 90% effort for the fast interval and 100% effort for the hammer interval. The fast interval is faster than tempo, but it’s not an all out sprint. So it is something that you would want to be able to hold for around 1.6km (although here you are only doing it for 300m).
This workout is great for improving your short distance running and mimicking the demands put on your body on race day.