Bromine swings both ways when coupling with double bonds

Here's a little chemical example of my favorite dictum: polarization followed by attack, followed by depolarization.

In certain solvents, molecular bromine, Br₂, cracks apart into plus and minus ions: bromide and bromonium (note the cool use of the "ium" suffix for the positively charged ion):

Br—Br  —>  Br^-  +  Br⁺

Now Br₂ will add to ethylene to make ethylene dibromide, a useful pesticide, but the bromine and ethylene can't simply couple because it's forbidden. Instead, bromonium ion approaches ethylene, sucking at and polarizing her most available electrons (recall I deduced that ethylene was female here). Bromonium's negatively charged doppelgänger (umpolung!) then backside attacks the polarized ethylene, after which her electrons collapse into bromonium's clutches and the partners relax. Here's a visual: