Your team is attacking and you are high, near the edge of your box. A long ball is hit over your defence. It is going to stop just outside your penalty area, and you can get there before their striker.
You read it early and you went. Outside the box you play like an outfield player, feet only. A side-foot pass to a team-mate ends the danger and gives your team the ball.
Right to come, wrong to use your hands. The ball is outside your area, so that is handball: a free kick on the edge of your box and very likely a red card. Always know where the line is.
You can still make the save from here. But you could have got to the ball first, and now their striker has a free run at you in a 1v1.
Practise it with Sweeper Keeper, Close The Door.