A high backhand is similar to a regular backhand.
It is, as its name implies, released from a higher
point than a regular backhand. High backhands
often are released at or above the shoulder.

What are high backhands good for?
Marks often attempt to take away the low to
mid level throws (as these are more common).
The high backhand, therefore, may often
be easier to release without being blocked.
It is best used for throws that "break the mark"
and for throws that need to reach a certain space
with less speed. The high backhand usually
incorporates less arm motion than a regular
backhand, and can be released very quickly.

The high backhand has a tendancy to float
(it relies more on the spin of the disc than
the motion of the arm release). As such, it works
well when the receiver is running at full speed
and will obviously beat the defender to the space
that is in front of both of them. Because it is
travelling with less velocity, it is an easier
throw to catch (but as it "hangs" in the air,
it can also be an easier throw to defend).