selecting the still frame for a transition effect

I am trying to select the still frame for a 'pan down fast' transition in a project i'm doing on FCP X (10.2.1). I am using Yosemite 10.10.3.

When I click on the transition two yellow dots appear indicating from where the still frame will be taken for the transition effect. The yellow dots have the numbers "1" and "2" in them and they are connected to the clip in the timeline by a broken yellow line. I can click and drag the yellow dots to reposition them. When I do this, the broken yellow line becomes a solid yellow line. I thought that the still frame used in the transition would now be taken from the new position of the yellow dot but this does not happen. The yellow dot has moved but the still frame used in the transition remains the original one.

Can someone please tell me how I can select a new still frame to be used in the transition effect?

thanks