-How do Multi-search engines work?

From the explorations that I have undertaken, there appear to be 3 different approaches which are in operation at the moment:
A straightforward list of different search engines
Searches which take place one after another.
Searches which take place simultaneously
Each of these has their own advantages and disadvantages, so lets examine them in a little more detail.
A straightforward list of different search engines
These work by simply copying the appropriate URL for the cgi script onto the web page. This is not particularly difficult to do and the user then simply inputs the appropriate search term(s) into the dialogue box and submits the search. This is then run by the search engine in particular and the user is presented with a list of results in exactly the same way that they would if they had gone to visit that particular site directly.
The advantage of this approach is simply that you can reduce the amount of time you spend going from one site to another in order to complete your search.
It might also suggest other search engines for you which you had not considered using before. It is almost impossible to keep up with all of the different search engines which are available, and if someone is happy to do this on your behalf, it makes sense to take advantage of it.
However, the disadvantage of this approach is that, strictly speaking, these sites are (in my opinion) misleading the user. They are not offering a multi-search engine, but have simply collated the work of others onto a new home page. Thats not to say there is anything intrinsically wrong with this, and the Webmaster will have had to have done a reasonable amount of work to set the page up, but its really nothing more than a slightly more sophisticated list of links.
These are by far the most common sites offering a multi-search facility and examples of sites which take this approach are:
Find-It at http://www.itools.com/search/
Searches which take place one after another.
This is much closer to the concept of a multi-search engine. A site of this nature will usually have a single entry line where you input the search just as you would with a single search engine interface. You may then have the opportunity of deciding which search engines you the search to run under (usually from a list given in a check-box type situation) and the multi-search engine then transmits the search simultaneously to all of the search engines you have indicated.
Once the search has been run, the results will be displayed on the screen for you in a list, commonly separated into the results as provided by the different search engines. However, the main disadvantage of this type of approach is that before the list can be generated on the screen all the different search engines have to have sent their results back to the multi-search engine site. Consequently the speed of the search is dictated by the speed of the slowest search engine.
An example of this type of search engine can be found at:
Dogpile at http://www.dogpile.com/
Searches which take place simultaneously
This type of search engine is very similiar to the previous approach, the main difference being that searches do not have to wait until each search engine has completed its work - as soon as results are available from one search engine, they are displayed on the screen for you to view. The result is therefore that much faster, and while you are browsing down through the list of hits, others are being added to the page even as you view.
An example of the multi-search engines which take this approach are:
Ixquick at http://www.ixquick.com/