Good question, and different people will obviously have different takes on it.
Here’s my take:
imagine yourself creating your circle, for whatever reason you’re doing this, now imagine all of things you are connected to in the whole world, like strands of energy or threads radiating out of you, all connected to these things; with your hands, wand, ritual blade or whatever other tool – real or visualised – that you prefer, “cut” the threads you want to be free of during the working, focus on the ones you want on the other hand. For example, you may want to make a connection to your landscape, but not be distracted with interference from work, studies or your social life. You’d focus on the threads that connect to the trees and the land and so on and ritually “cut” the other ones. A simple hand gesture, like untying a knot and releasing the threads could work, too. Once you have done what you set out to achieve, revisualise the other threads and “retie” them – reweaving with your hands, so to speak. I prefer the tying and untying idea myself, but that’s just me.
The idea is to create a space in the circle that is a concentration of power on what you want and is free of unwanted distractions or influences.
Hope that helps.