The camera takes the infrared picture at night when you turn off the flash.
That's the trick with all night shooting devices, that they don't need the flash.

An infrared camera records infrared, i.e. not "normal" light. No flash is needed for infrared photography.
The flash can of course be switched off.

The camera uses infrared light at night, which is invisible to humans. However, if you look closely, you will see a red glow, which is normal. The animals notice the infrared, but usually it does not bother them.
You should not switch off the flash, because firstly it is not possible (as far as I know) and secondly your pictures would be black at night (it needs the flash for pictures at night).