I responded in another forum to a similar issue as follows:
The 1/2 synchronizer sleeve (in which the 1/2 shift fork rides) moves towards first gear sliding over the synchro hub starting the synchronization process (synchro ring will be indexed and the cone will start picking up the cone of the first gear) Once the speed is equalized the coupler teeth will engage.. These teeth flanks have a back taper to assure positive engagement lock whilst transferring torque..
JOG (jump out of gear) can be created with following main conditions...
1) not enough spline (dogteeth) back-taper on synchro ring and coupler..(Manufacturing quality)
2) not FULL engagement during the shift cycle.. (shifting too fast or shifter boot interference)
3) the first gear has a triple cone synchronizer system and the floating synchro ring might drag too much (particularly in very cold weather..)
4) shift rail detent ball springs too weak (broken or worn out)
Everything has to be in balance to avoid "hard" shifts, (you still want to shift smoothly from 1-2 etc..) on one hand and to avoid JOG on the other hand..
The negative back taper is designed to create a lock whilst torque is transferred, as soon as you coast (reduce torque) the lock becomes less and with a bump it might do a JOG .
Just do a slow shift (be sure it is fully engaged) into first and get moving, keep a constant acceleration and maintain a slight load on the shifter towards second gear.. You will feel the "bite" from the back taper...if something is not right you will detect an easy slide out..
Bring the acceleration to almost coast and try this again..with a slight force the shifter can be moved out of first..
Just trying to give some background music..
Hope this helps..