At first, I was confused as to your character's intent with the spear. You mentioned trying to tangle up Sigurd's feet, only in the most recent post did that transfer into cutting straight through his boots and severing tendons. 

'Its blade is six feet long, its handle two,' so it's eight feet long.

What you'd be talking about then would be a push cut, which would only be possible if you could actually push the blade. Last I checked the end of the spear was dragging along the ground, when in actuality for this angle to be plausible it would actually have to be about two feet into the ground.

Finally, your character is pivoting away on their right foot, which is certainly conductive to retracting the spear, but not at all useful in a push cut or a sweep. A pull cut might have been possible, though once again the angle would be a massive hindrance as you're targeting an immensely low point on Sigurd's body to try and bypass the shield and Sigurd is extremely close to your character.  

Ultimately, unless you're arguing your weapon has lightsaber like qualities it's not going to have sufficient force to do anything, provided it could even be manipulated into getting under Sigurd's shield, which is highly unlikely (as it is not idling as you say, but actually covering his left side while opening space for him to move his sword around.) 

Also, Sigurd really wasn't relying on anything, there is literally no historical precedent ever of a spear being used in that fashion in a harmful way, except for its heat magic which Sigurd is still unaware of. And a spear would be immensely more manoeuvrable than an eight foot lance. Does your character have superstrength after all? If that's the case I'd buy it, if not its humanly impossible, or at least highly improbable.