It demands so much core strength that when repeated five times, it feels like a mini-marathon right in the middle of a very long practice.
In Mysore, India where R. Sharath Jois directs the Ashtanga Institute students sometimes even laugh at how hard this moment can be when held for five breaths, five times in a row with a lift-up for a break between repetitions.
Sharath has been known to tease students with a "ffff....." sound just before saying five. If anyone comes out of the posture he says not to hurry and makes everyone hold a little longer. I didn't think Navasana could get any harder than in Mysore, but then I found this awesome core strength movement. The muscles at the core of the body need conscious activation in order to strengthen.
This variation of Navasana, Boat Posture, from the Ashtanga Yoga Primary Series is the perfect place to get stronger. I like to call "harder-than-Navasana", but I've also heard it called Urdhva Navasana. If you practice this movement you'll be ready for Navasana next time!