Crescendo and diminuendo

The essential detail is that both are gradual and both have a destination. A crescendo that reaches its target volume in the first bar and then sits there has not been executed; the shape should be paced across its whole written length, usually arriving only at the final note.

String players build a crescendo by increasing bow speed and arm weight together while drifting the contact point toward the bridge, the reverse for a diminuendo. Planning where in the bow to start matters too: a long crescendo begun at the tip on an up-bow has the frog's natural weight arriving exactly when it is needed.