Sweep or economy picking is sometimes defined as picking with a mimimum of pick motion. There are often misleading examples, or ones that are outright wrong, of how to accomplish this. An “o” is a string in cross section in the examples below.
Pick two strings in the following way, alternating down and up strokes. There is follow-through which interferes with speed:
up <--o- o down o -o--> up <--o- o down o -o--> up <--o- o down o -o-->
If instead you alternate in the way shown below, the follow-through carries you to the next string. Thus you have more control of a greater part of your stroke and less motion altogether:
down >o o up o o< down >o o up o o< down >o o up o o<
Always move the pick in the direction of the next note.
down >o o o o down o >o o o down o o >o o up o o o o< up o o o< o up o o< o o down >o o o o down o >o o o down o o >o o up o o o o< up o o o< o up o o< o o ... ...
So with sweep-picking you can have both the advantage of reduced motion and opportunities to use continuous motion. Over time, with practice, the difference in sound between your up and down strokes will become less and less noticeable, unless you are playing chords, because when you are sweep-picking you are not always striking down on the beat and up when not on the beat. That is why you can’t tell whether an accomplished player is striking down or up by listening.