How works the eleventh water breathing technique? In the last episode (20) you see Giyu Tomioka use an eleventh breath technique. He stopped the blood threates without doing anything. So how does it work? And it was said that there were only 10. So why didn't Tanjiro know about this?
So how does it work?


Eleventh Style: Lull (拾じゅう壱いちノ型かた 凪なぎ Jū Ichi no kata: Nagi?)

Giyu engages a skill that disables and nullifies incoming attacks. However, its effectiveness is limited as fast and numerous attacks can break through - Link


So why didn't Tanjiro know about this?


Tanjiro doesnt know this because the 11th style is an original attack created by Giyu Tomioka but somewhere in the episode they said that every user can be able to have its own unique extra form.