Without a doubt my last crash at Sentul was due to in-proper downshifting. Truly beginner mistake as yours truly is still a beginner. Coming fast to a corner, you can not have your attention away with for example a rear tyre lock. As Keith Code says you only have limited attention. Especially when entering a corner those precious attention shall be allocated toward preparing for a corner smoothly not correcting the rear tyre lock.
Downshifting is to match the engine RPM to its lower gear. And doing so in high RPM it is important that you downshift smoothly and you should avoid rear wheel hop caused by un match RMP and gear selection. There are two technique that are used in doing this, Blipping (or commonly called “blyer”) or Holding (or also called “slip”) the clutch.
Blipping is basically you rev the engine during selecting the lower gear and releasing the clutch. So the engine RPM will match the selected lower gear. I think I can do this OK now. When I switch bike to CBR from Ducati, I did have some problem doing blipping with the CBR. Sometimes I got sudden jolt from the rear tyre due to improper timing in revving the engine. But it is getting better now.
Slipping is to release the clucth slowly (or locally called “Pirit”), in doing so the engine will release the torque to the rear tire slowly, hence avoiding the rear tire lock. At Sentul R3 and R4 slipping the clutch often the prefered choice. Just about coming to R3 you downshift to 2nd and hold and slip the clutch all the way to about enter R4.
Slipping proven to be difficult for me, and time for correction is limited due to short distance between R3 and R4. The final result was: CRASH.
For me more practice is in order, and try to avoid Slipping or Holding clutch at this stage.