How can Haku perform hand seals with only one hand? In the Land of the Waves arc, it can be seen that Haku performs handseals with only one hand. That must be something special, because a.) even Kakashi was surprised to see that, and b.) that's never seen again somewhere (except I forgot it).

How is that possible? Is that a special technique? Was it ever explained in the manga/anime?
Ebisu explains (in Chapter 90) that to perform a ninjutsu or genjutsu, a shinobi needs to go through 2 steps.


Build up the necessary amount of chakra for that jutsu.
Control the chakra for that jutsu using various hand seals.


Haku's ability to do hand seals with one hand shows his exceptional skill in the chakra control (at step 2). 

There are other ways in which a shinobi's prowess with hand seals has been demonstrated. Senju Tobirama could use the Water Dragon Bullet Technique with just one hand seal, while it normally requires 44. Uchiha Sasuke has been able to use Chidori without any hand seals, and Fire Techniques with just one hand seal. (He needed more seals for both the jutsu when he started.)

The other shinobi shown to be capable of one-handed seals is Guren from the Three Tails filler arc. (Source: Narutopedia)