This spell must go off in the same round as another spell from another spell caster. What happens then is the wild mage attempts to contain the energy of the other spell caster in an extra dimensional bubble.
In order to succeed the wild mage must make an Intelligence ability check modified by the difference in the levels of the two casters. For example, Fe'larn (12th level) casts an Energy Containment that goes off on the same round that Dallor the Evil (11th level) casts a Fireball. Fe'larn receives a +1 modifier to his ability check. This is further modified by the level variation table for wild mages.
If the check fails the Energy Containment bubble collapses in on itself and the resulting spell is unleased at double its normal power. In the example above Dallor cast a Fireball, if Fe'larn fails his Intelligence check the Fireball is released as a 20d6 Fireball. (I know the book says the max damage for a fireball is 10d6, but this is one of the risks of being a wild mage).
If the ability check is made the spells negate one another and nothing else happens.