The disengage in prima occurs when someone tries to beat the point of your sword away using a mandritto or riverso. So basically anyone using standard Bolognese tactics.
I was under the impression that this was a full disengage, but in practice we got to the point where the attacker could always wound the belly. Sticking someone in the face while having your guts cut out isn’t my idea of a good idea.
Fortunately a couple of our newer club members figured out the trick. After you lower the tip to allow your opponent’s sword to go by you don’t proceed to the other side as in a normal disengage. Instead you thrust under his sword, constraining it from below with your true edge turned against this blade. He can’t bring it down without freeing it, but your sword is too high for him to come over the top.
Though Agrippa is more upright than Fabris, it looks pretty much like this:
I’m surprised that I didn’t see it before, as my hero Fabris uses constraints from below in his prima as well. I even see it in SCA spears (I refuse to call anything under 12 foot a “pike”).
Second Technique
The alternative to this is to step directly to the right, catching the blow not near the point as your opponent intended but rather near the forte in seconda. It will naturally slide down into the guard, giving you a strong constraint.
It looks basically like this, except with a step to the right instead of a passing step to the left.
Grauenwolf's Study of Western Martial Arts 