Say the uids of all the objects in the groups are 0,1,2,3,4,5.
Then right before event 8 the "selected object list" or sol is all the instances in both:
group1= 0.1,2,3,4,5
group2= 0,1,2,3,4,5
Then in event 8 the condition "group1:pick by uid 1" modifies the sol to:
group1= 1
group2= 0,1,2,3,4,5
and the action "group1: set opacity to 50" takes the picked group1 or instance 1 and makes it's opacity 50.
Next consider event 9.
Right before it runs the sol is:
group1= 0.1,2,3,4,5
group2= 0,1,2,3,4,5
Then in event 9 the condition "group2:pick by uid 1" modifies the sol to:
group1= 0,1,2,3,4,5
group2= 1
but then the action "group1: set opacity to 50" is the same as in event 8. It takes the picked group1 or all the instances (0 to 5) and makes their opacity 50.