This upgrade consumes the target's Brittle condition on the user's next basic attack with Cleave activated. This in turn removes any other benefits of a persisting Brittle, namely +50% critical damage and the damage bonuses that can be achieved with other talents that exploit Brittle.
The stun effect is somewhat redundant, given that all spells that inflict Brittle will inflict freezing or paralysis along with it. Thus, the best time to use Claymore to stun would be right before the condition wears off.
The additional 5 seconds to Cleave's effect allows for roughly 8 extra hits at +75% damage (with no attack speed boosts). This translates to a 16.67% increase in damage across 15 seconds compared to using a non-upgraded Cleave for 10 seconds followed by 5 seconds of un-enhanced basic attacks.
At any critical chance > 0%, the additional damage from scoring critical hits on a Brittle enemy with non-upgraded Cleave over 10 seconds is roughly equal to the damage output of Claymore over 15 seconds, which consumes Brittle after one hit. This technically gives the basic Cleave a higher DPS, with the caveats being that the enemy must be under the influence of Brittle and the Brittle effect must last the entirety of Cleave's duration. However, it means fewer hits that have to weigh against enemy damage resistance to accumulate roughly the same damage, and it means fewer seconds of using nothing but basic attacks to maximize the talent's damage.