As yet unrecognized by the International Olympic Committee, EHO is nevertheless the sport of wily webmasters worldwide.
Users have short attention spans.
Human factors research has shown that users will wait at most 8 to 10 seconds for your page to display. That's the limit of people's ability to keep their attention focused while waiting. About 30-40K total at current bandwidth. That's where EHO comes in. EHO is more of an attitude than a specific technique.