The block (and armor) effects generated by enchanted items normally last the whole exchange.
However, blocks generated by evocation (and enchanted items) are 'broken' if the attacker gets a higher result.
Also, you still get to roll your normal physical defense (usually athletics) while protected by an evocation block.
Examples:
(We'll use you ring)
Our hero has athletics 3, is wearing the ring and is under attack by two thugs (weapon: 2 pistols, Good(+3) gun skill)
The first thug shoots at our hero, the thug rolls (+, -, 0, 0) for a total of 3.
Our hero rolls athletics(+3) to dodge and gets (-, -, 0, 0) for a total of 1.
He'd normally take a 4 stress hit here. He wants to avoid that, so he uses the ring to generate a power 4 block.
The power 4 block is able to beat the thugs attack result (of 3), so our hero isn't hit after all.
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Now, the next thug shoots at our hero, getting (+, 0, 0, 0) for a total of 4.
Our hero rolls his athletics (+3) and gets (+, +, +, -) for a total of 5. He rolled well enough to dodge the attack. But his evocation block is broken by the second thugs attack, so it goes down, even though our hero managed to dodge the attack.
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Notes:
Your evocation block can be broken, even if you dodge the attack.
You can't make people easier to hit by 'protecting' them with low power blocks.
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2nd exchange
The first thus shoots at our hero again, and gets (+, +, +, +) for a total of +7 to hit.
Our hero rolls athletics (+3) and gets (+, +, -, 0) for a total of 4.
He uses the ring to generate armor and takes a 3 stress hit.
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The second thug fires at him and gets a total of 3.
Our hero defends with athletics and gets a total of 3.
The armor effect is still around, and reduces the hit the hero takes to 0 stress.