In English, a doorway is also called «a bridge page», which clearly explains the main task of such platforms — to redirect the search traffic to the corresponding page.
This method of attracting visitors to the website is quite effective, as doorways quickly get to the top search results for low-frequency and medium-frequency thematic search requests. This happens first and foremost due to the large amount of keywords in the text.
Since doorways are considered to be spam (and rightly so), search engines are trying to pick them out and remove them from the search results as quickly as possible. In order to stay afloat for a while and have time to increase the traffic, doorway owners use all sorts of tricks such as hidden content, substitution of page content for web crawlers, etc.
For those who use doorway generators — dorgens, ban isn’t a real problem, as the program will automatically (or almost automatically) create a whole new bunch of doorways.
Dorgens vary in functionality, but first and foremost, any doorway generates unique texts filled with keywords. This is achieved through such methods as Markov’s chain, merging of snippets from the results of search requests, machine translation or synonymizators.
The latter usually result in a completely incoherent text, where instead of ”they do not understand each other“ it may say ”they friend each other not understand”.
Such content is used on black doorways. In this case, the text is seen only by the web crawler, and the user is automatically redirected to the specified page.
Gray doorways include pages with more or less readable text, which has links to the main resource inside. Web crawlers are less aggressive towards these kind of sites.
Those who need to quickly increase traffic, without resorting to black SEO, use white doorways. These are full-quality sites with unique content and are, as a rule, advertising the main site.
We do not want to give assessments to the doorways phenomenon. In the end, the choice of ways to increase traffic is always up to the webmaster.