Shinai has sides

The 'sides' on a shinai - think it like the sides of a coin. The Japanese term Ura (reverse) / Omote (front) collates nicely with such concept.

You need to be able to distinguish each side when you do below techniques:

1. 'Harai' (knock away)
Self-initiated, straight-line horizontal slap (to create an opportunity)

2. 'Suri-age' (slide up)
Reactive, upward curvature slap (to counteract already-happening attack)

Understanding the meaning 'Front (Men-side)' and 'Reverse (Kote-side)' may become relevant when you apply these techniques with subtly, such as the opponent's built & habit.

If you know the meaning 'front' or 'reverse', you can reason your tactics better because the muscle required for the 'front' side and 'reverse' side action are different.

Ideally you should be able to perform above techniques from both sides, but the Men-side tend to work better in certain circumstance and the Kote-side in another, and you need to figure out by yourself what works best for you, because aside the opponent's habit, you have your own habit/strength for each side and you need to know your own forte in order to make split-second decision for the best possible outcome.

Some opponents hold shinai tilted towards Kote (to guard), then slapping from the reverse (Kote-side) is easier as the distance to catch the opponent's shinai with yours is shorter, whereas if the opponent is holding the centre very strong, slapping from the front is effective to 'break' the defense and get that ippon with Men.

So, be aware of the both side and try to figure out which combination (opponent's attributes + your own habit/ability) works best, the strategy is effective only for you.