Multiple speedlites

I'm getting into some off camera flash work and am just curious. I'm not using TTL, setting my 580 to manual and have a YN460 to use as a 2nd for fill flash etc.

Why would you set flashes to S1 & S2?

Thanks.

what is S1 and S2? (probably a dumb question ....but hey, it's late...so cant' figure what it is....)

Sorry, Slave 1 and Slave 2.

The Yongnuo flashes have 3 modes. M = manual operation triggered via the flash foot or sync port. S1 activates the optical slave sensor and it will fire when it sees a flash. S2 activates the optical slaves but the flash will fire on the second flash it sees. This makes it ignore the preflash of an eTTL flash.
If you have your 580 in manual mode then set the YN 560 to S1 mode.

Thanks Dedsen.

This will just give different lighting effects?

No, this is just different triggering methods.
If your 580 is in eTTL,auto, Mode it will fire preflashes to measure the amount of light it thinks it needs. These preflashes will make your slave flash fire to early and out of sync with the camera shutter. Placing the slave flash in S2 mode makes it ignore the preflash and fire in sync withe the main flash. When you have the 580 in manual mode you are setting the power and there is no need for the preflash. You set the slave to S1 so it fires with the first flash it sees.

Fantastic. Thanks again I appreciate it.

Now to get practicing =)

Thank your from me too, very helpful.
I got a MK 951 and have been learning slaving it from the 60D. It's a good technical challenge for me and a learning curve on mastering light.