What is the technique behind mkv. video format?
This question is a bit nonsensical as Matroska is only a container format, however I think it does bring up a deeper question on how video files work in general.  Matroska doesn't actually define the way anything inside it is encoded or encrypted, it just stores the information that says what was used on the content so it can be decoded.

While Matroska does support encrypted content streams, generally video and audio is encoded, not encrypted.  Encoding is when a format is used to store data in a more efficient manner than a simple representation of the data, such as MPEG2, h.264, etc.  Encryption is when a key is used to protect data from unauthorized access or tampering and uses algorithms like DES, 3DES, TwoFish, Blowfish or AES.  In the special cases where video is encrypted, it is also generally still encoded prior to encryption to make it a manageable file size.

Either way, it is kind of a moot point though, because matroska can use any of a large number of encoders and/or encryptors.  As an open standard, the details of the container format are publicly available on the Matroska website here.  More precisely, this page describes the different codec formats that are currently suppoted by the container.

As far as the differences between Matroska and other formats, there isn't any meaningful difference.  It is a different way of storing the information about the streams, but the streams themselves are exactly the same as if they were in an AVI, MOV or m4v file (provided they are encoded in a codec that is supported in those containers.)  Transitioning between containers with appropriate tools is a lossless operation as the media stream itself is not materially altered.