Break loose = wheels sliding, lock up = wheel sliding also
There are plenty of times you have to change gears while still cornering. I was watching some DTM today and there was at least 2 corners on the track the drivers had to upshift while not entirely straight or during some sort of transient. They wouldn't have the throttle flat out, but they were still accelerating fast enough to require changing gears whilst still powering out of the corner. When changing gears if you dont match the revs well, the car will jerk, if the car is already at the limit of traction, that jerk can easily cause the rear end to break loose.
Try playing GTR with a wheel that has a clutch. In the game, try playing with both automatic clutch and manual clutch. Using the auto clutch, the game automatically rev matches when you change gears, its far easier to drive the car on its limit, whether it be braking at the limit or cornering at the limit. LFS is similar, but I can't remember off the top of my head how you set up the clutch in that game.
I know its realistic because I've seen people driving our FSAE lose the car while trying to downshift.