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This question gets asked so often I thought it should be addressed,
under a relevant subject heading that will show up during an archive
search.

The DIR approach is to dive with a balanced rig, in such fashion as to
enable a gentle kick to get you to surface in the event of a loss of
buoyancy.  The idea here is simple, but requires the correct gear in
the first place, to work.  This means not using drysuits made of
neoprene or other material where the buoyancy varies with depth, and
using the correct tanks for the application, so you don't require
massive amounts of inflation to offset the excessive negative buoyancy
(use only aluminum stages, and never use steel tanks when wetsuit
diving).

To develop a balanced rig, start by determining the amount of integral
weight that you need.  This will be the amount of weight that you
require to enable you to sink, with full tank(s).  This weight
essentially just offests the buoyancy of the suit and insulation, and
includes your tanks, backplate and V or P weights in some cases.  As
the dive progresses, you will consume the gas in your tanks, and get
lighter.  You need to be able to maintain your depth at the deco stops
when your tanks are empty, so you add droppable weight to compensate
for the weight decrease due to gas consumption.  Droppable weight is in
the form of a negatively buoyant canister light, or a weight belt.  The
only purpose of the wing is to compensate for the weight of gas in your
tanks.  Thus, barring any problems, you will start the dive with a
slightly inflated wing, and end the dive with an empty wing.  This
means that the majority of the lift capacity of your wing is used only
for emergencies, or keeping the rig afloat when you take it off on the
surface!

With a rig that is balanced in this fashion, you will always be able to
get up when you have to.  The worst case scenario is that you have a
complete wing failure at the beginning of a dive, when you are
heaviest, and your drysuit fails too.  In this case you lose the
droppable weight to bring you back to neutral and kick up.  You can
also move some of the integral weight to droppable, which will result
in positive buoyancy if you lose the droppable weight, but has the
advantage for the ocean divers (if in the form of a weight belt) that
you can take the rig off on the surface without fear of losing it.

One commonly asked question when wearing a weight belt as droppable
weight, is should it go over or under the harness?  The answer depends
on what the greatest risk is for the diver, ie. accidentally losing the
belt and becoming positively buoyant at the risk of blowing past
mandatory deco stops, or attempting to ditch the belt when you need to
get up, only to have it get hung up on your crotch strap.  The former
seems to be the greatest risk on any dive that requires decompression
stops, especially when you consider that should the latter problem
occur, you can still remedy it in a few seconds, you have the drysuit
as backup, and even if that had failed, provided your rig was balanced
in the first place a gentle kick is all that is required to halt your
descent.  I would only wear the belt on the outside if the ability to
ditch the weight was the primary concern, such as diving in swift or
aerated water, or any single tank, non deco diving.  If you do use a
weightbelt with a DIR harness and backplate, you may find that bullets
or curved plates positioned at either side of the belt will prevent
interference with the bottom of the plate.

The idea of a balanced rig does not wholly apply to wetsuit diving, due
to the compressibility and variable buoyancy of the wetsuit material. 
When wetsuit diving, the droppable component of your weight must
compensate not only for the weight of consumed gas, but for the
increased buoyancy of the suit at the surface.  You must necessarily
inflate the wing at depth to compensate for this.  This is a good
reason to only use aluminum tanks when wetsuit diving, to avoid the
excess negative buoyancy of the steel tanks.  Restricting the use of
wetsuits to relatively shallow depths will minimize this problem.

Do it right, or don't do it at all.

-Sean


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