This is just stupid.

[swirly]

As I understand it, BitTorrent is specifically set up so that the more you put in, the more you get out.
It therefore seems strange to me that restricting your upload speed could possibly increase your download speed,
...
Can anyone shed some light on this

Okay, so what happens when you try to download a file with bittorrent? Well first it has to find people who have pieces of that file. This means sending out some packets, thus upload bandwidth usage. In addition to this it has to find which of those people have pieces that you don't currently have, which means more upload bandwidth usage. There are also people querying you to see if you have any files and pieces that they need. When your client replies to them it has to use upload bandwidth. So you have all this information that bittorrent has to be able to send in order to work and actually I'm sure there are lots of bits I'm not thinking of off hand.

So what happens if your upload capacity is maxed out? Well none of that information is able to be sent. Thus you can't connect to people and thus don't get any downloads. Now as somebody stops uploading there will be a bit where your upload capacity isn't maxed. So then you might get a connection to a person for a piece or somebody else might start uploading from you. In this manner you will get some small amount of downloading as the information slowly makes its way through the congestion that is your upload. If you limit your upload so that its not maxed out, then the needed information can be sent without any problem and thus you are able to get downloads going much better than you would otherwise. If you think about it in that kind of way you should be able to understand why limiting your upload can actually make you download faster.