Canon EOS Rebel T6 Instruction Manual page 200

3 Setting the FlashN
Flash mode

When using an external Speedlite, you can select the flash mode to
suit your desired flash shooting.
Shutter synchronization

Normally, set this to [1st curtain] so that the flash fires immediately
after the exposure starts.
If [2nd curtain] is set, the flash will fire right before the shutter
closes. When this is combined with a slow shutter speed, you can
create a trail of light such as from car headlights at night with a more
natural feel. With E-TTL II (auto flash exposure), the flash will be
fired twice in a row: once when you press the shutter button
completely and once right before the end of the exposure. Also, if
the shutter speed is 1/30 sec. or faster, first-curtain synchronization
will be applied automatically.
If an external Speedlite is attached, you can also select [High-speed
synchronization] (e). For details, refer to the Speedlite's
instruction manual.
Flash exposure compensation

See "Flash Exposure Compensation" on page 117.
E-TTL II flash metering

For normal flash exposures, set it to [Evaluative]. [Average] is for
advanced users. As with an external Speedlite, the metering is
averaged for the entire area. Depending on the scene, flash
exposure compensation may be necessary.
200
[E-TTL II] is the standard mode of

EX-series Speedlites for automatic
flash shooting.
[Manual flash] enables you to set the

flash output yourself. This is for
advanced users.
Regarding other flash modes, refer to

the instruction manual of a Speedlite
compatible with the functions.