What it says in the title
How I imagine this to work is pretty much self explanatory but here:
One person wants to trade chests
One person will trade items( like key frags or keys etc) or the new BE/IP(when preseason hits) for those chests
They enter a trading window
Of course for people who are going to argue like "you can just grind one account when pre season hits, get loot from that and transfer"
Solution: both accounts must be online+level up loot can not be traded
Solution: Both have to solidify( basically make the items non retrievable from trade window) using a tick box, like Steam's trading service before trading