You want a torn, chiseled core that looks great when you take off your shirt and that gives you stability and strength in all situations. This core is built in two steps.
First of all, you need to build the muscles and strength below. You can't have six pack abs if you don't have basic core strength. This means that you have to hit your core from different directions. You need to teach your abs to turn, fight rotation, generally support yourself (think of planks), and bend your spine (think of situps). This strengthens the all-round strength that is among the best six-packs and the strength you need to fight your way through sports and lift things around the house.
The other half, of course: you have to cut off body fat. This means that you do some kind of conditioning training, push your heart rate up and challenge your heart and lungs. This strengthens your overall health and also helps the abdominal muscles.
Now you can do separate abdominal training and separate conditioning sessions to accomplish both goals and march toward the washboard abdominal muscles. Or you can attack both goals at the same time with an interval training unit, in which core work and conditioning ideas are combined in one training session.
Why core and conditioning pair go well together
You will not do as much pure HIIT in this training as you combine conditioning ideas with core training. And that works well because your core is the goal. The world has this endless love affair with high-intensity interval training sessions, but to be honest, HIIT doesn't work for everything. If you want to improve your bench press or really chisel out your arms at John Cena level, this is not the case with HIIT training. And HIIT also won't help you improve your squat numbers or dive a basketball, even if the mainstream fitness media tries to say so.
With HIIT workouts, you can't always improve technique in motion because, especially in the mass market version of HIIT, you are constantly out of breath and never really focus on technique.
However, mixing interval training and abdominal training is perfect if you are following a six pack. You can push your heart rate into the stratosphere with a conditioning movement (think of mountaineers or burpees), and immediately afterwards you do a concentrated core movement. You can do this because you don't charge aggressively in this workout. They mainly focus on body weight movements, so you have some leeway for mistakes.
The key to this training is not just surviving the core pull, but concentrating on the technique. Yes, you can make a sloppy plank for 30 seconds after doing a shipload of burpees. But can you make a clean plank with a narrow core and control your breathing? To get the most out of this training, you need to do the following.
Perform this training four or five times a week. You can do this as a standalone session or at the end of another strength training session. Remember to focus on the technique of core movements. Your goal is to precisely balance the overall fatigue that you feel after the conditioning moves during the core movements. It's sloppy to deal with, but then you can't get the most out of this session.
During this training you work 40 on and 20 off intervals and combine a conditioning train with a core train. First do 40 seconds from step A and then rest for 20 seconds. Then do motion B for 40 seconds and rest for 20 seconds. Do 3 sets like this (so you'll spend six minutes on each pairing) and then move on to the next pairing. Remember to focus on the technique, especially the centerpiece. Don't confuse a lot of sloppy repetitions with good work.
Bergsteiger: Start from a plank position and own the plank. Then hit these climbers.
Hollow Rock: Tilt the climber into the position for a hollow rock and continue hammering on the ideas of the core strut.
Situp Roll Burpee: Combine the sportiness of the Burpee with the classic situp for roasting in the abdomen.
Russian twist: Now create a rotation through your core with the classic Russian twist. Use a light load or light weight.
Kettlebell Swing: This movement increases your heart rate. Don't go too hard though, because you work for time, not repetitions, and focus on shape.
Panel Plank Push Pull: Take a panel or object that you can slide and pull over a carpet. Get in plank position; Work 20 seconds on one arm and 20 seconds on the other. Focus on keeping your hips and shoulders straight.