I have a discord video that explains exactly what i mean. https://cdn.discordapp.com/attachments/288528119525146635/634072701585195038/you.webm Basicaly the video starts off lasting only 0:02 seconds but as it plays the end time increases all the way up to 1:03 or something. I dont Know how people do this.
The video simply doesn't signal a length at all (i.e. a length of 0) - this way the length shown by different players can either be zero, how much is buffered, or even the negative length played (totem).