They modified the code to run under FastCGI. This is a chunk of the announcement from the MT developer's list:

This should be a particularly attractive option for ProNet members who need to launch sites for clients that want to get up and running with MT in a hurry: You just sign up and go, and there's no installation required. Plus, a bunch of popular plugins (such as BigPAPI, StyleCatcher and a WYSIWYG editor) are included in the install.

 
We'll also talk more later this week about the work we've done with Yahoo to help them run Movable Type under FastCGI. As those of you who've experimented with FastCGI know, this means MT runs a 
*lot* faster, with our benchmarks consistently showing improvements of up to 150%.


On 12/12/05, Robert Douglass <rob@robshouse.net> wrote:
"Six Apart said it had optimized the underlying software in Movable Type
so that it responds twice as fast as the same software offered by Six
Apart's own Web site."

This would imply that the optimizations are possible because of Yahoo!'s
configurations and environment, and wouldn't be possible for the general
software, no?

Dries Buytaert wrote:
>
> Yahoo! is about to offer MT and WordPress blogs.
> http://news.yahoo.com/s/nm/20051212/wr_nm/media_yahoo_blogs_dc
>
> --
> Dries Buytaert  ::  http://www.buytaert.net/
>
>