Copy and pasted from grok
High likelihood against anyone with solid grappling knowledge (BJJ, MMA, etc.), moderate against pure strikers, and a classic high-percentage setup overall.
In a ground-and-pound (GNP) scenario—especially from mount or high mount—trying to post (plant your hand on the mat or against the opponent) or fully extend an arm to block/stop a hook creates an isolated, extended arm that the top person can easily attack with an armbar.
Why this happens so readily
Your natural defensive reaction to punches is to push on their chest/hips, frame, or reach up to block. This takes your elbow away from your body and often extends the arm.
From mount, the top person simply overhooks or controls that arm, climbs to high/S-mount if needed, steps a leg over your head, and sits back into the armbar. The punches themselves are the bait that forces the exposure.
BJJ/MMA teaching emphasizes keeping “T-Rex arms” (elbows glued to your ribs) precisely to deny these isolations. Extended or posted arms are repeatedly called out as the primary entry for mount armbars.
Even if the top person is mainly striking, a grab on the extended arm can lead to an armbar if they know the mechanics—or at least put you in a worse position while they continue pounding.
Factors that raise or lower the risk
High risk: Opponent has any real BJJ/MMA training, is in full mount, or is deliberately mixing strikes with submission threats. This is one of the most common and high-percentage ways armbars are set up in MMA when GNP is happening.
