[Vidic]

Flat-footed can be a term used to describe a fighter who is not agile on his feet (i.e. not on his toes) If you are literally fighting flat-footed i.e. margarito you will have trouble pivoting and moving laterally

Some people are flat-footed while they punch (Cotto) but can be on their toes while moving

And some fighters (Foreman) are completely flat-footed, and have the soles of their feet planted while both moving and punching.

Planting your feet is a term used to describe slugging it out and standing your ground with your opponent, as it allows for a sturdier base and less mobility (theoretically it's easier to wind up shots aswell) as a rule fighters who punch genrally don't pivot while they throw, it's something common with 'huge hitters'

Leonard - Duran I - Leonard fought flat-footed

Leonard - Duran II - Leonard fought on his toes