The pistol squat and its difficulty

Hey ho a quick question.

I've found that the pistol squat can be made incredibly tough (way beyond my current strength) by keeping my torso vertical throughout the entire movement.

By instead allowing the torso to lean forward, the pistol squat becomes rather simple. I could do ten reps if I am allowed to lean forward, but not a single rep if I make sure to keep the torso vertical.

So - what is correct form? I can't find torso lean addressed in any pistol squat progressions.

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