I will say I'm sorry if I'm wrong, but I think you are confused about how the uploading works. Adding a magnet link from say TPB into webtorrent to check if it works is true. Where I think you might be wrong is after that. Copying the magnet link from the webtorrent client is a different link than the magnet link on another torrent site like TPB. I have actually tested it out by adding a new movie on the site and playing it. The downloading on the site ONLY comes from my link, meaning I would be the sole seeder until someone else could download the webtorrent magnet link. Then there would be more seeders. You cannot use torrent seeders for this.
I think the reason for it is this
Say you are downloading a torrent from a standard torrent client and a webtorrent client and you map the beginning and end of how the file is downloaded on both platforms, it would look like this:
WebTorrent:
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
A B
Straight across in a stream as that is the only way to handle a stream
Standard Torrent client:
|||| | ||||| | || |||| |||||||||| |||| | ||||
A B
Standard torrent clients cannot stream as it takes the most available pieces of the content until it equals 100%
This means, until you allow others to download those webtorrent links, you would have to rely on the original seeder/poster of the webtorrent link. They cannot remove that link and they will have to seed it.
WebtorrentDesktop, Vuze and a number of other torrent clients are what are considered 'hybrid' clients and can download and upload data between both traditional torrent protocol and webrtc based torrent protocol. By using a torrent link for a file already seeded by hundreds or thousands of seeders it makes the file more robust in the long term. For those who want to use a magnet link from our website to save the file (using a hybrid or traditional torrenting client) the file will be fast with tons of peers and seeds. For those who want to watch directly in the browser, their service gets faster and more reliable as the site grows and the technology is adopted by more and more torrent clients. It is worth mentioning that webrtc torrents are new but growing quickly. WebTorrent-Desktop has over 250k installs and vuze has well over a million. As they become more popular and as our site grows and matures it will be only natural for other torrent clients to add in webrtc compatibility
By suggesting you submit torrents to the site that are already popular, distributed and compatible it future proofs the service.
The in-browser streaming has limitations based on what browsers will and will not allow you to do. You cannot create tcp/udp connections by ip/port meaning you cannot connect to traditional torrent clients. WebRTC solves that restriction by allowing you to connect to hybrid torrent clients and other users of the site.
In regards to the magnet link changing: WebTorrent-Desktop adds a number of 'default' trackers to every item added to it to improve compatibility but there is a common misconception with torrents that the magnet link is needed. All you need for a torrent to work is the file hash. If you look at the magnet link you will have a parameter as urn:btih:. The hash following that can be placed in any torrent client and it >should< be able to re-create the torrent file. The trackers are there to improve speed of reconstructing the torrent file and to speed the process of finding peers where DHT cannot be used effectively or as quickly.
Very short version: When submitting to the site, please use media that is already well seeded to avoid issues. Right now the site is very much proof of concept still and we are working on the underlying code to make it as quick and stable as possible. In the long term we will rely on more torrent clients adopting the hybrid model which will offer a HUGE boost to site performance.
Now for your actual question: There's no technology difference in how torrents are actually run. Only the networking method to send and receive data changes. What happens is that it sets the priority for the 'next block' as high and as soon as one block is finished (generally around .5% of a total file) it will begin playing the video. When you seek to a different point in the video timeline it does the same thing simply re-ordering the block priority until the entire file is finished downloading. At that point the full video file is stored locally in your browsers temporary files and we offer you a link to save the full video file for later. Because you've already finished downloading it, saving the video file to your computer should only take a few seconds as it simply has to re-write it to permanent storage.
We have been culling old torrents users submitted who were incorrectly assuming our site would mirror and host the content for them (we do NOT host any copyright materials, that would be a huge legal issue for the project). As part of our next major set of site updates there will be a report video option as well as automated video analytic on our side to locate and hide/remove invalid torrents. At a glance it looks as though the items you linked had 1-2 seeders total so when they stopped seeding the video files became unavailable. This is why we HIGHLY encourage people who submit content to the site to choose an existing torrent rather than making their own. If you are going to re-encode something for compatibility sake, make sure to also submit it to major torrent sites so that there will be dozens of people mirroring the same content to ensure it is around for months/years to come with a proper swarm of seeders.
In your example it seems to work for me although it is a bit slow.