Also see the Moonies and the Scientologists for this kind of way of doing business.
The basis for this kind of scheme is not necessarily the outright purchase of a development, it is more a stampede of as many cattle throught the gate as possible, then brand them and move on to the next field. You can then get a discount from the developer, which you don't pass on (in theory you do, but in practice you charge a fee that wipes out the discount) You also get a commission from the developer as well, so the promoters do very well on this kind of high pressure sales technique.
The 30 minutes thing is what is called in sales parlance "getting to yes". A few people wil drop out, but not many, as once a buyer has made a decision the sale is effectively made and just has to be closed.
I would find such schemes an insult to my intelligence, but they seem to work all the time for the companies using these sales methods.