You get your big hardcover Rulesbook and study up. You have read four other editions of this game and you've learn they changed the rules on you. Like when they had artillery dice. When you would add d4+d6+d10 to your weapon damage. When there would be -9 to your armour save. So you think you have the new rules down and head out to game. Only for your opponent to bring something up that sounds wacky and implausible. So you look through the book or as I do hand the book to them to find it. Yep, they are right and how come you didn't catch that the first time?
So I'm creating posts to help me remember the game so I'm more proficient next time.
Two rules that came up recently.
Armour Penetration pg 60 in rulebook. This came up when using Monolith's Particle Whip against Skarbrand who was an inch in front of it. (Thanks, Suijin, for the suggestion to provide context)
"The centre of the blast marker ends outside the vehicle, but part of the marker covers its hull. In this case the shell or missile has missed the vehicle, and only some shrapnel clanks against the vehicle's armour. The armour penetration roll is resolved against the Armour Value facing the centre of the marker, regardless of the position of the firer, and the weapon's Strength is halved (round down).
Going to Ground pg 24 in rulebook. This came up when I asked Joe x during our game of Noir Engels vs Tyranids. His hormagaunts went to ground to get a save from rapid fire bolters. I asked if a unit that went to ground can attack in assault. He replied "I don't know."
"If assaulted, the unit will fight as usual, but gains no advantage for being in cover because they are not set to receive the enemy charge (see page 36)."
If there's an enemy unit in cover and it goes to ground it does not get to attack first if your squad doesn't have frag grenades.