The concussion mine resting there for more than ~6 seconds grants Junkrat 2 further concussion mines in his pocket. When an aggressor, in this case the Echo, blitzes Junkrat, this enemy will likely get trapped. Concomitantly, the Junkrat should detonate the resting concussion mine, and then detonate a pocket mine shortly thereafter for a double-jump to gain significant vertical elevation that will serve to disorient the enemy (they're generally not familiar with tracking a double-jump, compared to a single-jump), and accordingly give Junkrat the time & space to land a slam dunk to close out the duel.
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