How do they achieve this?

[sunny_s]

http://www.airtel.in/cricketonyoutube/

[Rust]

The ISP is simply raising the speed cap on accounts for a specific URL (http://www.youtube.com/ipl) during a specific time period.

Their expression "no buffering" is simply marketing speak for "you won't notice if it buffers for 2 or 3 seconds".