Spacebands are also called "bands in space" or "BiS".
A spaceband usually tends to pull its starting segment toward the point in space during wiggle and other operations. A spaceband can also push the starting segment away from its end point in some cases.
A very short spaceband tends to keep the starting segment at the same spot.
Recipes can draw spacebands using precise geometry using the band.Add function.
Bands drawn by band.Add are slightly different than manually drawn bands. Bands drawn by band.Add have a default goal length of 3.5 Angstroms. The same default is used for bands created by band.AddBetweenSegments.
Manually drawn spacebands have a goal length of zero.
The goal length of any band can be adjusted manually in Foldit by right-clicking (or control-clicking) the band and selecting "Change Band Length". The function band.SetGoalLength can also be used to change the goal length.
A spaceband always starts on a specific segment, and really a specific atom, of the protein, and ends on a point in space. A goal length of zero means the spaceband tends to pull the protein until the start point reaches the point in space. If the start atom reaches the end point, the band would have an actual length of zero.
A spaceband with a non-zero goal length may push or pull the starting point, depending on its current actual length. For example, if the band's actual length is 10 Angstroms, and it's goal length is 20 Angstroms, the band tends to push the starting point away from its end point.
The actual length of any band can be seen by right-clicking (or control-clicking) the band and selecting "Change Band Length". In the Change Band Length dialog, "Use Current" sets the band's goal length to is current actual length.
The goal length of a spaceband is also shown by a small cone along the band. The start point tends to move closer to the cone during wiggle. When the goal length is longer than the current actual length of the band, the cone appears at the end of a thin rod projecting from the end of the band, indicating a "pusher" band.
All of the same length considerations apply to bands between two segments. Bands between segments have two small cones to indicate their goal length. Setting the goal length to zero for a band between segments would tend to cause severe clashing as the atoms at each end point approached each other.