SQM is like an alien Star Trek spaceship. Imagine you're driving down a freeway, and you hit one of those irritating spots on the freeway where there was an accident 4 hours ago, and because of that, there's a 2 mile backlog of cars. The traffic is free and clear AFTER the previous site of the accident, but leading up to that spot, it's going super slow, and you have to sit in this line of cars all moving at 2 miles per hour. This is like a buffer backlog.
Now imagine an emergency vehicle needs to get through. It needs to get to the other end of this backlog FAST or it will be too late to do any good.
There are two ways we could do this:
Prevent the backlog from happening in the first place... by beaming backlogged cars back to their origin using the star trek transporter... Without a backlog, it's easier to maneuver and packets don't have to wait in a long line.
Keep an "extra lane" open for emergency vehicles... Only they can use it, and they fly through without delay.
SQM is like both of those strategies put together. It drops packets, which is like an alien spaceship that comes along and just beams cars back to their house. It looks at this enormous backlog of cars, 2 miles long, and maybe it just beams 2 out of 3 cars back to their original starting point... Suddenly there's room to maneuver on the freeway, everyone can put their foot down on the accelerator, and within seconds everyone's moving at full speed... Everyone who's been beamed back to their starting point just has to set out again and start driving, but they're not congested either, so they go at high speed...
This is the basic theory, but then in addition to that it's possible to use something like diffserv4 which is like extra lanes... If your packet has a "flashing light" or an HOV lane sticker (ie. a DSCP tag) it can move into one of the reserved lanes, and skip the wait.
That's almost literally how it works. It keeps all the packets flowing by dropping packets which is like sending them back to their starting point, and keeping lanes open for high priority stuff.