When in doubt, keep it simple until you get the hang of it. That is, use the bridge as a message to the animal that they have performed correctly and will be rewarded. This IOU, when applied correctly, will build confidence in an animal and avoid confusion. Normally a bridge is used to reinforce the best response or peak of a behavior. The fastest, best responses are what you will want to reinforce.
Types of bridges can vary, sometimes the bridge is a word, a whistle, a tone, or a clicker.
It can also be a flashing light or another visual signal. Bridges can also vary in length of presentation and in their placement. The best placed bridge, for reinforcement, is at the peak of the performance. The important thing is to be careful with how you use the bridge and to apply it consistently to the animal. Here is where things get confusing and more complicated.