If a bison can not see what awaits him in an area, there is a normal reluctance to enter - a matter of survival. On the flip side, most animals feel safe once they are in a dark area. If I can't see you then you can't see me. As mentioned above, bison feel secure in dark areas. The use of solid sides can greatly reduce the level of fear in bison while they are being worked.
Solid sides provide darkness as well as the illusion of restraint.
It is for this reason that a solid top prevents bison from climbing out of an alley or squeeze chute. This illusion of containment causes the definition of 'solid' to be a bit nebulous. For calm animals, solid sides only need appear solid, for excited or wild animals they must be solid. Conversion of an existing open sided facility to a solid sided facility can be done with minimal expense and materials. Cardboard can be temporarily used to enclose a squeeze chute or the alley leading up to the squeeze chute.