"Twist" is a deceptive term when used with respect to a saddle. It could
indicate a natural part of the tree, it could indicate a defect in a tree,
or it could be the width/narrowness of the saddle at the waist.
The tree of an English saddle is the structure on the inside. The tree
providess the basic shape of the saddle:
The seat (purple)
The points, which rest on the horse's shoulders (red)
The gullet, which provides clearance off the horse's back (blue);
the angle of the gullet determines whether the saddle is narrow,
medium, or wide.
The waist, the center of the saddle, and where the twist is most
acute (yellow)
The stirrup bars (green)
the "twist."
The twist of the saddle is literally the way that the tree conforms to
the horse's back. It must present a flat surface to the horse's back
all along the tree; at pommel, the points lie parallel to the horse's
back, which is a pretty steep angle. At the cantle, the horse's back is
much more broad, and the tree must be much more flat to remain parallel
to the back.
To get a clear picture of how the tree twists, hold your hands out in
front of you with your thumbs pointing up. Then pull your hands toward
your chest, turning your thumbs in. The tree twists in much that way;
the ssurface is much more vertical at the front, and much more
horizontal towards the back.
So that, literally, is the twist: the way the flat part of the tree
twists to stay flat against the horse's back.
The narrowness or wideness of the saddle at the waist (where the twist
is most drastic) is usually referred to as "wide twist" or "narrow
twist." The width of the twist is a comfort issue with most riders, but
the location and angle of the twist must also fit the horse.
The description of the waist can be deceptive in advertising, by the
way; a saddle advertised as "wide" or "narrow" may be describing the
tree itself, OR it may be describing the waist. If buying a saddle, be
sure that you know the width of the tree as well as the waist.
A "twisted tree," on the other hand, is a warping of the tree such that
it is no longer symmetrical. The warped tree creates pressure points on
a diagonal. The point will dig in on one side, and the tree will dig in
at the cantle on the other side.
A minor twist in a tree can be reparable, but a major twist may not be.
A crack in the tree is not reparable.