An overplay tries to gain too much.
If the opponent replies correctly, it will show a loss, but if he makes a timid response or other mistake, it can succeed. Most trick plays are overplays.
Overplay for learning
While overplays are mistakes in competitive games, they are good opportunities to learn. In particular, it is better to err on the bold side than on the timid side.
You don't learn much by playing underplays; you just lose a game by 10 points, and you aren't sure why. Playing overplays is instructive. When you get punished: you learn something. You'll never find the line between the two if you always play under it. And, if you overplay constantly, your overplays will get smaller and smaller as you learn, until you find yourself playing right on that fine line of "good play", or at least close to it.
Overplay and handicap Games
Consideration of what White does in skilful handicap play is revealing about the nature of overplays. For example invasions may be legitimate plays, may be overplays that should end in mochikomi, or may live with correct play but give away too much influence.
- In the opening White may play for large, loose frameworks on the assumption that Black will not invade correctly.
- Characteristic of play in the middle game in handicap go is that White makes invasions of the type that give away too much influence. That leaves problems for White's other groups in making shinogi.
- In the endgame White ought to be making correct plays and relying on Black's small mistakes to catch up. Speculative invasions are not good teaching, in general.
The true nature of handicap go with White is to be a little unreasonable, in a way that is hard to punish. I'd say obvious overplays are only part of it. White's natural strategy is of the amashi kind, but thinner than it ought to be.