Most people do Planks wrong..
Incorrect technique results in decreased strength gains with poor transfer to calisthenics training.
The 5 Worst Plank Mistakes:
1 – Lower Back:

The goal with the plank is to stress the anterior core muscles. Letting the lower back arch transfers unnecessary stress to the lumbar spine. We need to maintain a horizontal position to gain ab strength for calisthenics.
2 – Long holds:
Holding the plank for a long time is a waste of training effort. This is going to build endurance at best and is generally coupled with form breakdown. Effort is better spent using a harder progression such as the long lever plank.
3 – Forward Head:
The gooseneck is a common form mistake. Aim for a horizontal neck position, this will strengthen the neck flexors and extensors.
4 – Not Breathing:
Holding the breath during planks is useless. Forceful breaths in and out creates a stronger ab contraction and is more relevant to a conventional training situation.
5 – Hips High:
Sticking the ass up is another way of cheating. The stress is being transferred from the core to the lower body making the exercise easier.
You should now have a better understanding of how to plank correctly. Make sure you’re not making any plank errors!