Diamonds uses the Variable Strut Inclination method as described in EN 1992 §6.2.
The inclination of the concrete strut is expressed with the angle θ and can vary between 1 ≤ cot(θ) ≤ 2,5 (45° ≤ θ ≤ 21,8°). Diamonds assumes θ is at least 31° (reason: increasing anchoring length). The inclination is increased when the concrete compression stresses in the compression strut are too high.
The inclination of the stirrups is expressed with the angle α. α can be 90° (vertical stirrups) or 45° (stirrups under 45°). If the compression strut is still insufficient with stirrups under 90°, Diamonds will switch to stirrups under 45°. Stirrups under 45° are represended in Diamonds as vertical stirrups + longitudonal reinforcement left and right (NL: lijfwapening). the vectorial sum of both is equivalent to stirrups under 45°.
Beam 4m simply supported on both sides, 200x400mm, concrete cover 40mm, C25/30, 25kN/m dead load, 10kN/m life load (Diamonds-file in the attachement).
From equation 6.13 in EN 1992-1-1