From D&D 3.5 to 5.0(5E) - 26 - Aid Another to Give Your Buddy an Attack Advantage

If you feel like your character can't roll high enough to hit this monster, but maybe the Paladin standing next to you can, then you can forgo your attack action in D&D 5.0 and instead choose to use the "Help" action to help the Paladin hit the monster on her attack.  Simply tell the DM that you are going to put your thumbs in your ears, wiggle your fingers in the direction of the monster while doing a little dance.  If the Paladin then choose to attack the monster before your next turn, the Paladin will have advantage on her first attack.

From D&D 3.5 to 5.0(5E) - 25 - Non-Lethal Damage in 5th Edition. A Knock Out!

You won't find non-lethal damage in 5th edition, but when it comes time to strike the killing blow the attacker can show some restraint and "knock out" their opponent instead.  This only applies to melee attacks.  But if you character is one of those that wants to show mercy, or you want to tie the monster up before waking it up to interrogate it, then a "knock out" is for you.  Simply tell the DM, at the time you deliver the blow that would have put the creature at zero hit points, that you would like that damage to knock them out and leave them stable.  As the book says, the attacker can make this decision the instant the damage is dealt.