This is the king of the leg exercises in my opinion. Very few people reach this level of conditioning so if you can do this you get kudos from me. I learnt this exercise from Convict Conditioning and starting from step one, I worked my way up gathering strength to be able to execute this exercise.
For those who think this looks easy, I challenge you to try do 2 sets of 50 reps, which is the elite stage in Convict Conditioning. Admittedly I cannot do this. For the progression system and exercise background check out Convict Conditioning.