Honestly the traps have to be relaxed and stretched to get a full lat spread in the rear lat spread. You can raise the shoulders a bit and make an effort not to round the shoulders and thoracic spine too much. Doing that too much puts them at an angle that almost makes them invisible. Also on stage or any time the camera is lower you need to arch your back more, remember you are showing people below you the muscle so hunching forward is going to lose a lot of the visibility of well everything you are trying to show off. I was told to act as if I had a flash light on my back and was trying to point it toward the judges or camera. However as far as traps go in a rear lat spread, the reality is that if you want to spread the lats as far as possible you have to relax the traps and let your scapula come out away from the center line of your body. Trying to contract the traps will pull them in and you end up having a much narrower spread.@MrKleen73 any tips or tricks for keeping the traps popping while you’re in a full lat spread??
A rear double biceps is where you show your traps, or the rear "relaxed" stance you can contract a bit as long as you already know how to drop the lats as that should show off more of the back musculature but a rear lat spread is only to highlight the total width of the lats, V taper, lower back for upper body and then the best calf, and separation between the quads and hamstrings. Only people with massive traps are going to show much trap in a rear lat spread.
What most people do to show the traps and back detail when going into the rear lat spread is give the inner back a good squeeze showing off all of that musculature then put their hands on their hips and spread wide into the lat spread.