Good questions! I hope that I can offer a reasonably accurate and understandable answer...
Back in the Good Old Days when it was all settled by horsepower and guts; you shoved your right foot as far forward as you could stand and hung on... But it got to a point on the larger tracks where the speeds were getting pretty dang scary. (over 212mph!) It all came to a head when Bobby Allison got launched up into the catch fences and tore down a whole bunch of it in a very spectacular incident. In order to reduce the speeds, they put "restrictor plates" between the carburetors and the manifolds to cut the horsepower from about 850 down to around 500... Now nobody had the power to get out in front and get away; if you tucked in behind somebody they found that 2 cars were faster than a single one, and a whole pack was faster than just two... For about 25 years they've been trying to come up with the proper balance between horsepower and aerodynamics so that the racing would be more than single-file freight trains...
This year they gave a little more power by enlarging the restrictor plates, and also made the rear spoilers shorter... (They stick up into the air flowing over the body to restrict speed and provide downforce.) Now they run much faster, but with less downforce on them, they can get a bit "unsettled" when things get dicey... :shocked:
The "Bump drafting" originated when the cars were running nose to tail all of the time. Rules this year affect how long they can stay in that sort of positioning by changes in the cooling sytems and rear bumpers; you'll now overheat if you stay behind somebody too long. A tap on the rear bumper to nudge somebody along has been done ever since two or more cars lined up. This year they have found that it you tap somebody on the left rear going into a turn, it's a recipe for all kinds of mayhem... :yikes: due to less stability at speed. You might have noticed the "Shark's fin" that runs down the C-pillar back to the spoiler on the left side... That's an attempt to provide more stability...