Comrades,
As trainers and students, we have all experienced our knee collapse during the first part of the Turkish Get-Up. Once we go to elbow to post, that knee always wants to cave in. There are many ways to fix this, but I found one that gives you immediate feedback.
Before attempting this, please use a spotter and a light Kettlebell.
Get into the roll to post position, on your back with the bell overhead. Whatever knee is up the door should be on the inside of the bent knee. So really the door is between your legs. Make sure the door is almost touching the inside of your bent knee.
Once you go to elbow to post, if you move the door you are collapsing the knee. Try to practice just the elbow to post with out moving the door. It will change your TGU. Most likely it will make it harder, because it's now making you use your heel, elbow and abs.
Try it and give me some feedback.