the rule :
A player with this skill is armed with something very good at stabbing,
slashing or hacking up an opponent, like sharp fangs or a trusty dagger.
This player may attack an opponent with their stabbing attack instead of
throwing a block at them. Make an unmodified Armour roll (except for
Stakes) for the victim. If the score is less than or equal to the victim’s
Armour value then the attack has no effect. If the score beats the victim’s
Armour value then they have been wounded and an Injury roll must be
made. This Injury roll should ignore all modifiers from any source -
including Niggling injuries. If Stab is used as part of a Blitz Action, the
player cannot continue moving after using it. Casualties caused by a
stabbing attack do not count for Star Player points.
So, in my opinion, you can use stab instead of blocking or use stab as part of a blitz.
And then, if you blitz and want to use stab, you should lost 1 Movement point, isn't it ?
but in my last game against halflings, Zara had 6 move to do to join her aim :
and has no GFI to do to use the stab.
Just tested again in solo game, I face the same behaviour, which is an issue according to mmy way of reading.
And I think that' should be the way it was thought because, if you click on stab first, then choose a distant aim here is what you have : you only have 1 remaining move possible (not 2GFI). but the roll is not done.