Practising the Lunge

Coach Orion Stone-McNeil and Assistant Instructor Evangeline Bisal looking at the mechanics of a lunge and practising the action in a brief working lesson during club practice.
The bread and butter of fencing attacks, the lunge is an essential tool, but regularly done incorrectly by beginner and novice fencers. Avoid lifting the foot like an advance and pushing forward from the back foot. Instead, kick your front foot out in front while pushing with the back in order to project your lunge forward rather than up. Don't forget to keep your front foot relatively close to the floor to ensure a speedy lunge as you don't want to accidentally jump upwards.

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After the student scores a touch, the coach sometimes taps the blade from underneath, would you explain the purpose of this cue and when is best to use it?