5 things.
1. Reactions/timing/ping
2. Turning while swinging *turning into the swing
3. Hiltslashing *moving very close while swinging + turning so the animation hits you sooner
4. Footwork
5. High wpf/agi builds and loomed weapons.
BUT. If you have decent ping less than 70 or so, you should ALWAYS have a chance to swing back after a block.
Practice!