Hi Opt 2 counting system is not as effective today as it once was when there were loads of blackjack games with one or two decks. Nowadays you'll mostly find shoes with six to eight decks and the Hi Opt 2 Count isn't as effective in those games.
When compared to systems like High/Low, the Hi Opt 2 Count is more challenging simply because of +2 and -2 values which are assigned to certain cards while the other mentioned systems only have -1, 0 and +1 values. The Hi Opt 2 system is therefore a "multi-level" blackjack card counting system.
Here's the Hi Opt 2 method:
|2, 3, 6, 7||+1|
|8, 9, A||0|
|T, J, Q, K||-2|
Worth noting is the value of aces which is zero. This is because there's a side count for aces which makes the system even more complicated. I'm not familiar with how the side count for aces works in Hi Opt 2 system since I've never used the system. More information can be read in Lance Humble's 'The World's Greatest Blackjack Book', which was the first book to introduce Hi Opt 2 to the world.
All-in-all this is a more challenging and not as straightforward as some of the other blackjack systems. Hi Opt 2 certainly isn't the first system you should learn, and I recommend starting from the Hi/Lo system and then possibly trying out Hi Opt 2.
The Hi Opt 2 counting system is a balanced system, meaning you'll end-up with a count of zero at the end of the deck/shoe. It's good to know this because when you practice at home, you'll know that something is wrong in case your score isn't zero at the end of the deck/shoe.