Too dry

Too dry and you risk cracking a branch. Too wet and sap flow creates branch pressure, also encouraging cracks.
Wiring, like every other bonsai practice is a delicate balance requiring learning, patience and practice.
Execution ...

It is so discouraging to lose a tree in one day, after months or years of care, because it got too dry, but it happens to all of us. In a cool, humid climate, our little forest could remain outdoors whenever the temperature is above freezing.

If the soil available to the roots of a plant is too dry, root hairs wither and are no longer able to absorb water and nutrients.

If the sponge gets too dry you can loosen the cap a little to release the vacuum and more herbicide will be released onto the sponge.

The Madagascar Palm needs to be kept slightly moist as it loses leaves if it gets too dry. All parts of the plant are poisonous if ingested. The use of a humidity tray will keep moisture levels up.

very slow grower, especially in pots; needs shade; wiring not used with; yellowing fronds are old or Manganese deficient; evenly spaced leaf spotting means too dry between waterings; side shoots root easily; ...

You should reduce watering before repotting
because dry and loose soil makes repotting easier to
carry out. It can also reduce the damage to the
roots. You should complete repotting quickly lest
the roots become too dry.

more standard, orange-coloured, clay is a good substitute to use. It has the added advantage that its colour will change ever so slightly as the moisture content varies. This gives a good visual aid to determine whether your Bonsai soil is too dry.

In the winter months you won't need to water nearly as much but should still be closely monitored and not forgotten. The best way to keep a bonsai from becoming too dry or too wet is simply careful observation and proper placement.

If you cannot ball it, it is too dry, and if it does not crumble, it is too wet.

This helps make the branches more flexible, but take care not to let the bonsai become too dry as this can be damaging to the tree
thick branches that are to be bent drastically should be wrapped with several layers of raffia for extra support.

See also: Bonsai, Plant, Tree, Growing, Trunk