You switch tasks. Part of your mind stays behind.
The researcher Sophie Leroy named this attentional residue: leave a half-finished email for a meeting, and some focus is still stuck on the email.
Because that leftover attention is busy elsewhere, you meet the new task with less than your full mind.
The cost is real: slower thinking and shakier focus for the first minutes after the switch.
So finish a small task, or park it with a clear next step. Less residue follows you into the next thing.