If enabled, attempting to attack a friend or heal an enemy will instead change your target to your target's target and then activate the ability.
The way that reads, if I'm there with Bill fighting Tom and Jerry - if Bill's got Tom targeted and I've got Bill targeted, if I trigger say SNB - it's going to switch my target from Bill to Tom and then trigger SNB. If I've got Jerry targeted and Jerry's got Bill targeted, if I trigger say TSS - it's going to switch my target from Jerry to Bill and then trigger TSS.
The typical MMO thing doesn't actually switch the targets - it just attacks the target of target or heals the target of target. Did it work that way in STO? I honestly can't remember. I'm usually just setting the focus target, then when it switches the target for whatever reason, I switch back.
A thing that RIFT did that I thought was cool was having both Offensive and Defensive Focus Targets. That way, in a non-team scenario where I might not have access to hitting a function key to select somebody - I could still set that primary target and the healer if they needed some love.
Then again, along those lines, something I miss from EVE is locking multiple targets. I've always found it odd that while STO will track all the targets, you can only lock one at a time and then if you want to switch, you've got to hunt them down by seeing if somebody on the team's got them locked - tabbing through - or hoping that you can click them without causing cursor tails and desync.