How do I take this backlit shot? - is it even possible?

What kbal is talking about is Hypersync, not HSS.

I keep trying to read about it online, but I do not quite understand what it's doing. Supposedly it fires the flash before the first shutter starts moving instead of after the first shutter stops moving. Not sure how it extends the flash duration though.

At least for my strobe, there are two tricks..

1. at full or near a full pop so the duration of the flash is long enough to have light for the full shutter movement

2. having something put the camera in HSS mode so it will send the 'fire' signal at the correct time.

Number 1 is dependent on the strobe flash duration.. some strobes can't do Hyper sync as they are too short even at full power.

Number 2 can be done with a HSS flash on top of the camera or a YN-622 on top and setting it to HSS in the flash menu.

(my discontinued strobe also has an FP mode, so it does have a sort of HSS, but most strobes dont)

Heath wrote in post #15858232
What kbal is talking about is Hypersync, not HSS.

I keep trying to read about it online, but I do not quite understand what it's doing. Supposedly it fires the flash before the first shutter starts moving instead of after the first shutter stops moving. Not sure how it extends the flash duration though.

The flash have a short peak at max intensity. Depending on design, you can then remove the power to it. Or you can continue to keep the capacitor connected to the flash tube and have the still remaining energy create a "tail" of weaker and weaker light. This tail is much weaker than the main flash "explosion", but can be light enough and long enough that it can illuminate your scene for the full length of time that the shutter slit moves over the sensor.

So HSS gives many short but strong pulses while the shutter slit moves over the sensor.
And Hypersync gives longer lasting, but weaker, light while the shutter slit moves over the sensor.
While "normal" flash is a single very strong flash pulse that is so short that it only works if the shutter time is low enough that the shutter is fully open when the flash pulse arrives.

Hypersync requires that main pulse to happen before the shutter opens, so all that remains is the much dimmer tail when you burn the last remaining energy from the capacitor. The trick with this is that the flash picks up the "pre-flash" signal from the camera - a request by the camera to emit just a tiny, weak, flash pulse just to measure the light level. So the "pre-flash" signal makes it possible to activate the flash at "negative time" - before the photo is taken.

Since HSS and Hypersync will always light up the full scene but the camera will only have the slit see a smaller part of the scene at a time, you will only be able to capture a fraction of the light that the flash sends out. A shorter shutter time means the shutter slit will be narrower. So less amount of flash light will actually be captured. Just as less ambient light will be captured.

For "normal" flash use, the shutter will open fully before the flash ignites. So the full flash power can contribute to all parts of the sensor in that single moment of light. This is why the amount of flash light captured doesn't change if going for a longer exposure time - the exposure time controls amount of ambient light, while the dialed in flash power setting controls amount of flash light captured.