This is written in the spirit of “Thinking in public”
It is by no means an authoritative guide, but more of a snapshot of my (novice, 85% lead) current understanding of the idea. Feedback & contributions are welcome!
Lead projection in West Coast Swing is the technique of foreshadowing the upcoming movement (pattern) through positioning within the slot, adjusting the pitch, rotation, etc at the end of previous movement.
Lead Projection is also sometimes referred to as "pre-leading" or "slot dynamics".
You create a projection by adjusting your position, pitch or rotation at the end of preceding pattern (~through last 2 counts of it).
Projecting Direction
People often talk about slot as "train tracks" - you can imagine leader stepping onto one of the rails or staying in the middle as a way to indicate the direction of next pattern to the follower.
If leader is in the middle of train tracks - blocking the way of the follower - that implies a blocking pattern is coming (e.g. sugar push or sugar tuck).
If the leader is off to a side - they are inviting the follower to pass them on the other side.
Example: leaders offsetting to their right at the end of the pattern invites follower to pass on their left (e.g. left side pass) for the next pattern.
Up and down (pitch) projection
Another thing you can project is whether next pattern would have a higher or lower focus.
Higher focus usually involves turns of some kind (sugar tuck, underarm pass, inside roll).
Patterns with lower focus is when your connection point stays approximately at it's default height (sugar push, left side pass, whip).
You indicate a higher focus by:
Projecting Rotation
You can think of projecting rotation as a form of prep that is shifted earlier in time/incorporated into previous movement.
Doing projection as a follower
You can influence the characteristics of the next pattern as a follower by basically doing a mirror version of the things described above for the leader.
So to request a direction - you'd offset yourself to one side of the slot at the end of the previous pattern (it's important to do it at the very end because if you do it in the middle - leaders will often match you/square up).
And to request rotation - you'd prep yourself in advance.
Lead projection is not necessarily something you do deliberately. Sometimes it happens that you naturally end up, say on a side of the slot as you're completing a movement (or over/under-rotated).
People often have an instinct to "square up" to their partner, as they anchor, in such a situation (negating rotation, moving to the center of the slot).
But you can also interpret that as "naturally occurring" projection, and pick a new movement based on the position in the slot and an angle you've developed. Arguably doing this can create a better "flow" in the dance.
Make sure to project only things you intend to, avoiding miscommunication and noise in the connection.
One mistake I've been making as I started to incorporate the projection into how I lead is to stay "square" to the slot when offsetting myself to one side of it. That in turn would have an effect of rotating the follower - giving them rotational projection when I was not intending to do that.
To avoid doing that and to just communicate a linear intent (e.g. when leading passes) - you want to keep the point of connection stable and within the slot, while offsetting your body to one side of it.
You can choose to project rotation as discussed above, but it's important to only do that intentionally.
When leader is directly in front of the follower at the end of the pattern (blocking their way through the slot)
When the leader is offset to the left side of the slot
A lot of this is inspired by Joel Gibson's notes on the topic.