Topline is the muscular development from the poll, down the neck, along the back to the top of the dock which enables the horse to use himself properly.
Slowly is how I did it - long reining, lots of hill work in walk, and then when he was strong enough work in the school and lunge work with side reins.
It does depend on the breed, general health, conformation and mental attitude to work though.
thanks i thought this too but i cant have been the only one whos unsure
i also do this by work on the lunge with side reins and schooling on a correct contact