These are commonly used to create 'doors' in forcefields.
In the GUI, you can change the position of the tube to;
This will mean the tube is generated with the X radius on each side of the projector.
This will cause the shield to extend in X blocks of the projector. You can increase the length up to 32 blocks, and/or the radius up to 5 blocks.
Usage as an Entrance
If the tube projector is on the same core as another projector, then the holllow interior of the tube will override the other forcefield; allowing you to create an entrance in a shield without switching it off.
As the projector is in the middle of the tube, some entrances can be partially obstructed by the projector itself.
If the tube projector is powered by a different core, then they will intersect like normal and not open a gap in the field, so you cannot force a way through someone else's shield without a Frequency card from the same Core.