All the gers can only go as fast as a maximum speed for that gear. Therefore, if you are shifting down without braking enouigh, or slowing down enough, you will be going too fast for the gear you are in, causing the back wheel to lock up.
Yup, it's called engine breaking, useful in four strokes when just chopping the throttle though a little different. You need to practice when to down shift i.e...after just the right break use or coasting. If you just slam it in lower gears you are putting a lot of unnecessary load on you engine, though through practice you will be able to get it just right. Loose soil or off cambers will amplify the effect.
Ive heard all this ranting and raving about 4-strokes compression braking, and i tried it on my cr125, downshifting and poppin the clutch. Sorry, very little effect, i guess your need more PSI of compression, as in a 4 stroke or 250/500? I watched the 4 strokes use engine braking, followed by a bwaaapp, and a bucket full of sand hitting me. Ouch!, and I was wearing a chest protector:( . But using the disc brakes by themselves lets me concentrate more on getting th feel for them and brake on the threshold of traction rather then lock them up and use compression braking which would be worthless in that situation.
A few years ago when I was racing a 4 stroke I posed the question to Mike Young if engine breaking was something he used or if it was more of a nuisance. He advised me to pull the clutch and use the brakes for pretty much the same reasons FMXN gave.
The best use I ever had for compression braking was to drop the front end when merely letting off the throttle in the air.
hope i'm not off topic, but it just hit me why i clipped the top of the last jump in the tripple,i think i' got off the throttle and dropped the front, i found myself doing this a couple of times later. i knew about comp braking( my crutch i have to over come) and i knew about using the rear break to bring the front down, but i never made the conection with comp breaking:confused: learn, if ya live long enough.