The darce is weird because it can seem undefendable from that position when you're a blue belt and below, but then can seem unfinishable from purple belt and above. Aside from pre-emptive actions like using your offhand to block the crossface, and making sure there's minimal space between your head and opponent's legs/hip, you should understand the mechanics for finishing the darce. The choke works by forcing your own arm into your carotid, similar to an arm-triangle. So if your in-choke bicep is away from your neck, either with arm extended or elbows tightly framed, then you cannot really be choked. Your opponent also needs a certain angle to force your arm into the choke, and I feel like lower belts often relinquish this angle because they feel the darce being threatened and then freeze up. Watch here how Garry Tonon eliminates the threat of Keenan Cornelius' darce by going into deep half guard: