Most mindfulness instruction distinguishes formal sitting from informal practice, then explains the first in detail and the second in a sentence. The second is doing specific work that is worth stating.
It is defined by context, not by content
Informal practice means applying the same attention during activities that were going to happen regardless: walking to a bus stop, washing up, waiting for a lift.
Nothing is added to the day, so the instruction is about how an existing activity is attended to rather than about finding time.
That makes it available to people whose circumstances rule out a quiet twenty minutes, which is a substantial share of anyone learning this.
Frequency does different work than duration
A long session trains the ability to stay with an object under stable conditions with few competing demands.
Many brief moments train the retrieval of that ability from within ordinary activity, when nothing has been arranged and no timer is running.
Skills learned in one setting transfer poorly to another unless practised across settings, which is the strongest argument for doing both rather than choosing.
Ordinary activities are chosen for a reason
The recommended activities are routine and mildly repetitive, because such tasks run automatically and therefore leave attention free to wander.
They also recur daily without prompting, so the practice attaches to something already reliable instead of depending on remembering.
Pairing an intention with an existing routine is a well-established way of making a new behaviour stick, and informal practice uses that directly.
The difficulty is remembering, not doing
The action itself is trivial. The failure point is that a person intends to attend to their commute and arrives at work having thought about a meeting.
Which is why teachers suggest anchoring the intention to a fixed cue such as a doorway, a kettle or the moment of sitting down in a vehicle.
The cue does the remembering, and the practice only has to survive the few seconds after it fires.
What it changes over time
People who sustain informal practice tend to report noticing reactions closer to the moment they occur, rather than reconstructing them afterwards.
That shortened gap is what makes any different response possible, since a reaction identified an hour later has already run its course.
The change is undramatic and accumulates slowly, which is precisely why informal practice is undersold in materials that need to demonstrate a result quickly.