Transfer and expansion in recurve: shift load from arm to back, trigger the clicker with 1 to 2 mm of micro-movement, and what the research says.
The honest answer up front: transfer means shifting the load from the arm into the back, and expansion means opening the back a touch further until the release happens by itself. In the DSB model, transfer is a "very small, consistent rotation of the draw-arm elbow", and expansion a micro-movement of 1 to 2 mm that triggers the clicker [1]. For comparison: that is less than the thickness of your little finger.
What does the research say? In an EMG comparison of 30 men, elite archers showed clearly higher activity of the lower trapezius on the draw arm, and lower activity of biceps and triceps than beginners and pre-elite archers [2]. The authors conclude that the lower trapezius fixes the shoulder blade. In plain words: the pros pull with the back, not the arm. In 39 World Cup archers, a shorter reaction time to the clicker, higher draw force and less body sway after the release also predicted higher scores [3].
Both studies show associations, not causes. The numbers are also not a recipe for your bow. The DSB model is a teaching model. In addition, the EMG study examined only men, and the World Cup study elite archers with a lot of experience. What applies to beginners is therefore not proven. Take them as a direction, not as proof.
How hard is it? This is a pro skill. In experience terms you need many months of clean repetition, and an on-screen description does not replace a coach's eye.
At anchor the index finger sits flush against the jawbone, the string touches the corner of the chin and the tip of the nose. The model gives 90% back tension and 10% forearm tension for the anchor [1].
Now comes the "very small, consistent rotation of the draw-arm elbow", the transfer [1]. The load moves from the arm into the back. Milestone: your coach sees on video that the elbow rotates and does not drift outward.
Back tension and support line are held, in the model with "90% + x" back tension [1]. You aim without letting the back go slack.
While aiming you increase back tension once more. The back of the upper arm moves backward "parallel" to the shooting line, and the contraction in the back widens the chest. This is the micro-movement of 1 to 2 mm that triggers the clicker [1]. You do not pull further with the hand, but with the back.
With the click, the balance of forces dissolves and the string slides through the fingers [1]. Milestone: 10 shots in a row where click and release happen without a conscious finger command.
Measure your time between anchor and release. In a study of 15 archers, however, five timing factors together (pre-shot time, aiming time, follow-through time, arrow speed and the variation of the time to the clicker) explained only 7.7% of the variation in arrow scatter [4]. Anchor: a small but real lever, not a magic trick. More important is that your sequence takes the same time in every shot. For 10 shots, note your felt aiming time and compare it with the video: anyone who aims for a different length every time also shoots differently every time.
In transfer the load moves from the arm into the back via a small rotation of the draw-arm elbow. In expansion you open the back further and trigger the clicker with it [1].
According to the DSB model a micro-movement of 1 to 2 mm [1]. That is small. If you need a lot of travel, anchor or draw weight is usually the issue.
In a study of 30 men the lower trapezius was more active in elite archers, biceps and triceps less [2]. That is a single finding, not a training plan.
In 39 elite archers a shorter clicker reaction time predicted higher scores [3]. A calm, constant sequence counts more than frantic reacting.
Hinge Release: Letting Back Tension Fire the Shot