The finishing position is different on both the picture and the animation? I'm still confused on correct form for maximising rear delt work.
Ooops! The animation next to the text is performing a bent over row, not a rear delt fly. The upper arm bone is moving back, not outward.
FWIW, take a look at this:
Dumbbells are raised by shoulder transverse abduction, not external rotation, nor extention. Upper arm should travel in perpendicular path to torso to minimize relatively powerful latissimus dorsi involvement. This mean at top of movement, elbows (not necessarily dumbbells) should be directly lateral to shoulders since elbows are slightly bent forward. To exercise posterior deltoid and not lateral deltoid, keep upper torso close to horizontal. Positioning upper torso at 45° is not sufficient angle to target rear deltoids. The spine can be flexed to achive this positioning if thighs can provide sufficient support for torso. Some individuals may not be able to bend sufficiently at hip due to flexibility or girth constraints. Also see Rear Lateral Raise Errors and Low Back Alignment Exceptions.