Electrophilic addition of bromine

[Clari]

Hi! I am stuck in revising the electrophilic addition of bromine across the alkene molecule... There are two sentences which really confuse me, I hope you can help me.

When a bromine molecule approaches the pi electron cloud of the C=C bond...the bromide ion then attacks the pi complex nucleophilically from a side opposite to the first bromine. This becomes known as a trans attack and the result is a dibromo product.

My question is why the bromide ion have to attach to the pi complex from a different side and not the same side so that leads to the cis attack?

Thanks in advance!

 

[GCT]

Due to steric effects, you'll want to observe the electron density of this reaction.