Wyckoff trading method, page-1436

The breakout bar itself ??
or the how strong the market should be after a breakout ??

On the actual bar, it needs to show 'intent', so it needs to stand 'tall and strong' above the previous accumulation zone.
So the bar would have a wide spread, the close would be high, and volume above average, but not excessive (if volume is really massive or excessive, it may infer hidden selling pressure within the bar (pump and dump anyone ??), or it may show a large amount of supply has been drawn out from the left (stale supply), and this would suggest that the accumulation is probably incomplete, and may need to be extended before the markup is continued. I expect price would then retreat back into the relative safety of the old trading range, resume the accumulation process, and the push higher would be called a 'false breakout').

The potential strength of a market itself refers to the 'cause' that has been built.
As in "Cause and Effect"
An accumulation zone is said to be the building of a 'cause', and the effect is the response (higher) to how large or strong a cause has been built.
So the larger the cause that has been built, or put another way, the larger the accumulation has been, the larger the effect in response should be, or put another way, the stronger the market should be.

And just why I am on all this,
the three laws that are followed in VSA/Wyckoff analysis are-

Supply and Demand
Cause and Effect
Effort and Result

cheers

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