Edit: FINALLY fixed as of 3/17/2022 (both conceding during sideboarding and losing match due to roping out, and presumably to the chess clock running out as well)
Not just the “concede a lot for easy pairings” idea detailed in Inside the MTG: Arena Rating System, which still works quite well, as another Ivana 49-1 cakewalk from Plat 4 to Mythic the past couple of days would attest, but this time exploits that can be used for the top-1200 race itself.
In Bo3, conceding on the sideboarding screen ends the match *and only considers previous games when rating the match*. Conceding down 0-1 makes you lose half-K instead of full-K (Bo1 K-value is ~1/2 Bo3 K-value). If the matchup is bad, you can use this to cut your losses. Conceding at 1-1 treats the match as a (half-K) draw- literally adding a draw to your logfile stats as well as rating the match as a draw. If game 3 is bad for you, you can lock in a draw this way instead of playing it.
If you win game 1 and concede, it gets rated as a half-K match WIN (despite showing a Defeat screen). This means that you can always force a match to play exactly as Bo1 if you want to- half K, 1 game, win or lose- so you don’t have to worry about post-sideboard games, can safely play glass-cannon strategies that get crushed by a lot of decks post-board, etc.- and you still have the option to play on if you like the Game 2 matchup.
Draws from the draw bug (failure to connect, match instantly ending in a draw) are also rated as a draw. I believe that’s a new bug from the big update (edit: apparently not, unless it got patched and unpatched since February- see the comment below). It’s rated as a normal draw in Bo1 and a half-K draw in Bo3.