So, with Explosive Trap, if you have taken both the No Man’s Land and the Blast Rain nodes, you don’t throw traps equal to your maximum number of traps; instead you throw additional traps as if you had only taken the Additional Clusters node but you spend mana as if you had thrown your maximum number of traps and the skill does go on cooldown.
In other words, you spend the mana, the skill goes on cd, but you throw fewer traps than you should.