Focus: Hitting a bigger or stronger opponent
This is a drill isn't one of mine, I learnt from one of the Surrey Rollergirls coaches - Prof. Killer Hertz - who has a history in martial arts, thus has a knowledge of taking someone's balance.
Going in for a hit should be about using your biggest weapon against your opponent in the best scenario for you: your whole body weight when they are off balance.
Part 1: Groups of 2 - getting the opponent off balance. There are "windows" into the body that are either responses to pain or muscle reflexes and structural parts of the body, bone and ligaments/muscle groups. By hitting skaters here, this can place the opposition skater off balance. When done correctly the opponent should at least for a fraction of time be less stable than before.
Part 2: Groups of 2 - following through with the rest of your weight.. so an effective way of doing this is making sure you remain low when delivering the 'off balance window' then you can drive through your opponent by driving up and through them when you stand up more. Standing up into the drive means your body weight is coming from the strongest muscles in your body.. your legs.