Confusion remains...See [YS p.276]
If the attack surpasses the block strength of the ward, then the ward is breached; apply whatever shifts get through to the target just like bypassing a block (page 210). Alternatively, the attacker may apply those shifts directly toward getting rid of the ward itself; each shift will reduce the value of the ward by one until it’s gone. While a ward is technically still around at Mediocre (+0) strength, most lack the energy to hold themselves together at that point; a ward needs to be reduced to –4 to be completely nullified.
Now, if I understand, if an attack bypass the block value of a ward, the attacker can do two things: 1) apply the shifts towards a target behind the ward, and 2) apply the shifts to reduce the strength of the ward. But nevertheless, the ward is breached. Howerver, it does not seem to be "dispelled" yet.
Back to my "landmine" issue. Does it only go off the first time the ward is breached? Or when it is nullified (reduce its str to -4)?