Tricky one this, my feeling is that if you have the rear brake on, then that allows you to remove all slack from the chain, when you then put the power down you put the chain from "slack" to "tight" in a fraction of a second, thus putting a lot of force through it.
The opposite is brakeless, when you have a constant pressure on the chain, it doesn't have such a quick difference of state, so would be harder to snap.
just my 2p worth anyway