I won't make this an essay ( I have plenty of other thoughts and info on this topic) but...

The whole premise of altitude training is based upon developing an increased oxygen carrying capacity (through increased levels of heamoglobin) brought on as a response to decreased levels of oxygen.

This can be achieved in a number of ways including the obvious (1) living and training at altitude (2) breathing modified air from a device such as described by Leelee.

Another method of breathing modified air (ie decreased levels of oxygen/increased level of Carbon dioxide) is through a device called a snorkel. Due to the nature of a snorkel when you breath in and out on the surface there is residual exhaled air left in the snorkel each time. This residual air has a higher concentration of carbon doxide therefore a lower level of oxygen. When you breath in you are breathing in a portion of 'fresh' air and a portion of 'exhaled' air.

This has a similar effect as the regime that Atapene tried and a similar effect as breathing at altitude.

Perhaps one of the reasons that Liam experienced little or no effect from the modified air regime is that he was already conditioned to this type of breathing.

I experimented with this type of training many years ago by building a 're-breathing snorkel' for use when training on land (such as stationary cycling, stair-running etc). Essentially it was a SCUBA mouthpiece that was connected to tube in the shape of a standard snorkel (much shorter however) in one direction and the tube coming out of the mouthpiece in the other direction that wrapped around the back of my head and fed itself back into the 'main' snorkel again. That means I was breathing a much higher level of exhaled air than from a standard snorkel. (hmmm... that probably doesn't make much sense to anybody :? )

Anyway, did I have a reasonable breath hold? Yes. Did the re-breathing snorkel work? Don't really know for sure. No real way of measuring it coupled with other training I was doing at the time. A few years later I saw a similar device used by some elite athletes in the USA. So, maybe there was something in it.

There are also plenty of other things to do to improve your dynamic breathhold.