Why the name S30?
The Hambleton Bard S30 cylinder contains 240g CO2. It was named S30 some 30 years ago because it contains 30 times as much CO2 as a small 8g bulb (the small ones you can buy online and often in your local supermarket). The "S" comes from "seconds". One 1-second burst of CO2 from the S30 cylinder is approximately 8 grammes, so the S30 cylinder contains 30 x 1 second bursts of CO2 (240g). That is a bit of forgotten history behind the trademark and nothing really you need to know. The only thing you need to know, is that it works. It works and it is usually the cheapest and most convenient solution for putting CO2 into your pressure barrel.
How to use the Hambleton Bard S30 cylinder
Inject more CO2 into your pressure barrel when the beer no longer flows smoothly when you open the tap.
Simply screw the S30 cylinder on to the genuine S30 valve on top of your keg until you can hear the valve open. Let the CO2 flow for maximum 1 second at a time (or your valve may freeze up in the open position).
When not in use, remove the cylinder from the valve.
If CO2 continues to flow out through the valve even though you have released it - immerse the valve in warm water (not the cylinder body). You can simply put it quickly under a hot water tap. When the CO2 gas is injected, the valve will become very cold and the longer you inject gas, the higher the risk that the valve will freeze in the open position.
Why add CO2 at all?
Why do you need to add CO2 when you have added priming sugar to the beer, surely that will contain natural CO2?
Yes it will, but as you keep opening the tap of your pressure barrel and letting beer out, the empty space on top gets larger and larger and the available CO2 from the beer will evaporate more and more. To keep a constant CO2 level in your beer and to make it flow well out of the tap, you need to keep a constant CO2 pressure. So every so often you need to inject a little CO2 and this is most economically done with the S30 system.
Alternatives are to use either 8g bulbs which inserts into a special handle and can then be screwed on to the S30 valve with the pin inserted. This is a convenient, but not very economical way of doing it so although a perfect way to start, you soon realise that you need a cheaper method. That is the S30 cylinder from your local home brew shop. Buy it once, then just exchange it when it's empty.
The final alternative is more for the true nerds. You can use a Cornelius keg or similar pressure barrel and then buy even larger CO2 canisters. That is out of reach for most normal home brewers though but it is out there if you get really seriously into making (large amounts of) homebrew beer. It has a high startup cost so it doesn't fit the bill for most normal home brewers.