Joining in the middle of the game

Lets say 6deck deck game is 30% thru, can a card counter still be effective? Im just beginning to learn card counting and i havent tried it in an actual casino.

I cant clearly understand this statement from lesson 12:
Card counters 'scout' the tables in a pit, looking for a good count before sitting down.

... How can you look for a good count? because based from what ive read so far, im assuming a card counter should optimally open a blackjack table (start from a 100% shoe) because the true count is still zero. Thank you in advance.