Elmer Fudd said:
Can I Be thick here (no change there then!) Just how do clipless work ? Do you "snap" the cleat in and out, or is it like a twisting motion ?
Say I bought some dual pedals, say shimano spd's one side, flats the other, does this mean I am locked in to buying shimano shoes or could I buy any make as long as they accept spd cleats?
I've only ever ridden with flats + toe clips.
Basically there is a raised bit on the bottom of the shoe (this comes with the pedals and not the shoes which you bolt on to the shoe) that you push (or clip) into a recess in the pedal (binding). Some pedals are dual sided, some single sided and yet others are flat one side and clip-in the other side.
SPD's are a specific type of clip-in pedal which is generally considered a mountain bike pedal because this was the market these were created for and the major market that still uses them. Unrecessed clip-in are very slippy and hard to walk in and therefore not good for anyone needing to do a bit of walking off the bike without a change of shoes. For this reason many commuters will use spd's because of their versatility.
You may ask then why everyone doesn't use them for clipless systems. Well they have a much smaller cleat which means the loading on the foot isn't spread as widely therefore on longer rides some people experience hotspots which can be painful and make cycling difficult therefore those on long hard rides who are forcing every ounce out of each pedal stroke will opt for the wider cleat systems which aren't recessed as they have no plans to do any serious walking as this will help spread the load and thus avoid the problem with hotspots aiding performance.
SPD (Shimano Pedalling Dynamic) whilst it bears the name Shimano is widely known clipless system which many manufacturers now make compatible shoes for therefore you are not bound to buy shimano shoes (which for some of us are just too narrow to be comfortable), indeed I have specialized body geometry sport mtb shoes to use with my shimano pd-m324 (dual purpose) pedal (ie as you describe - flat one side, spd binding the otherside).
In order to release yourself from the pedals once clipped in you simple twist your foot outwards and they will release. Most pedals (if not all now) have tension setters which allows the beginner to set them loosely so they can break free easily whilst they get used to twisting out and as cleats wear so the tension needs to be increased to prevent accidental unclipping which could be dangerous (but then again no more dangerous than toe clips which are too tight). Whilst convention is that you should twist outwards for some people, such as myself you have foot probs, twisting outwards may not be an option therefore multi-release cleats can be purchased for most pedals which allow you to twist inwards or outwards as well as pull sharply upwards to release - all these options obviously increase the chance of accidental unclipping therefore tension monitoring will be more important as cleats wear so that this doesn't happen.
Hope that sort of answers your questions.