If you finish and say you managed to calm your mind down first, something has quietly slipped. You are not cleaning the water or blocking the flow, you are only sitting beside it.

What the research says

The best known meta analysis of self control pooled 83 studies and found that exerting control on one task left people worse at controlling themselves on the next, although later work has questioned how big and how robust that effect really is. Effortful control looks costly, which is a reason not to build your main practice on forcing a state. The questions raised about that literature mean this is a caution rather than a settled law.

Strength of evidence: one line of research. A single study, or a theoretical account.