M mode:

In this mode,you can adjust flash brightness according to your own preference,the speedlight will flash under the camera synchronous signal.

S1 mode:

In this mode,DF-400 will trigger flash with the main speedlight first flash,the effect is the same as using radio trigger.When using this mode,your main speedlight should be set to manual flash mode,using neither TTL flash system nor red-eye removal function which caused multiple flash.

S2 mode:

S2 mode is similar to S1 mode and is also known as “preflashing cancel mode”,but it can ignore the preflashing from TTL,so you can use it to support the main speedlight that working in TTL mode.In particular,if S1 mode is unable to synchronously flash with your built-in speedlight,you can try to use S2 mode