Effect of copper salts on conjugate addition

An organocopper can be made by transmetallating a grignard reagent. This also acts as a nucleophilillic carbanion:

Copper is less electropositive, so it produces a less-charged (or "softer") carbanion. And we know soft nucleophiles are a good way to encourage conjugate addition instead of direct attack. Example:

Another method to produce an organocopper is reacting it with two equivalents of alkyl lithium:

Which also has a soft carbanion. These are unstable and must be used immediately, this is why they are kept at low temperatures.

Reacting these happens much better by adding Me₃SiCl:

Though its mechanism is not fully understood.