The Seishogan's first special ability grants the user to see and sense chakra of the surrounding. Managan can see the chakra flow of a person when the eyes are open and can sense chakra of the surrounding when the eyes are close. Seishogan can see and sense chakra at the same time by closing and opening each eye, but the effectiveness will decrease.
The Seishogan's second most prominent special ability allows the user to absorb the chakra surrounding. Normally, when a shinobi casts a jutsu, a small amount of chakra will leak out. The chakra is invisible for the naked eye, but Seishogan users can see it. By activating the second ability, the Managan will automatically absorb the chakra surrounding into the user. Using this ability on the opponent's jutsu will decrease said Jutsu by one Rank (S-Rank to A-Rank, B to A and so on.. meaning that D-Rank will be completely voided.)
The Seishogan's third and the most powerful special ability is manipulating chakra of the surrounding. Seishogan user are capable of controlling the chakra surrounding. By controlling the chakra shape and movement, they can turn it into a deadly weapon. This ability drains the user's chakra significantly. That's why they use their ability to absorb chakra and store it, to have a very big chakra pool, which allows them to use this ability without any problems.