petey,

One of the key differences is before the start your proper course is pretty much any course you choose to take, so for example, a boat overtaking to leeward can deliberately luff, making it look like rule 17.1 does not apply. After the start a boat is constrained far more by rule 17.1 because her proper course better defined (though still moot).

There was a great animated resources at www.isafrules.com but it's either down or dead...

Another great resource is the ISAF casebook itself.

www.sailing.org/rrs2005/casebook2005.pdf