Tip of the day:
When using your flash outside, you have to be careful: you cannot exceed your camera’s maximum flash synch speed – normally around 1/200th second. This means in bright light you cannot use a wide aperture like f/4 (which after all might mean you would need 1/800th second, say, even at 100 ISO).
But if you have a suitable external flash you can exceed that speed (the flash pulses at 30 kHz-50 kHz instead of flashing all at once). If so, high-speed flash, or FP Flash, can be engaged on your flash.
On Nikon cameras, and on Canon cameras built after 2005, you can leave this on, and it will engage when the speed exceeds your flash sync speed, but it will not be used if not needed.
The drawback of fast flash: you get less effective power. Half at best, at smaller apertures much less. Meaning less flash range: but at least you can get outdoors portraits with large apertures.