Containment and cover is a time proven, effective tactic known as hammer and anvil. For raids, the "hammer" is the entry, while the "anvil" is the containment. A simple cause and effect that cause "fight or flight." While some suspects fight, most flee—opposite the entry point, usually out the back.
You go in the front, they flee out the back. This requires rapid and/or covert rear cover deployment to capitalize on the element of surprise. Consequently, rear cover usually deploys slightly ahead of, or simultaneous with, the entry—or suspects will likely escape.
Depending on circumstances and/or configuration, some raids require double perimeters to prevent escape. While it requires more manpower, this is a highly effective tactic—especially for "runners." It's even more effective when combined with K-9 teams.
Oftentimes, there is limited useful cover (from bullets) on raids. What do you do when there's no cover whatsoever? One tactic is to position yourself up close to the target itself. While this isn't actual cover, it prevents suspects from spotting you, unless they expose their position. Definitely better than being totally in the open.
A proven effective verbal command is to order suspects to "stay inside!" Then, broadcast this to entry. This keeps suspects bottled up inside the target. If they do come out, prone them out. And if "runners" get past the first perimeter, don't chase—or you'll leave your position uncovered. This is where second perimeters and K-9s are effective.