Transition Switching is a game grappling mechanic where a transition is started normally then is changed midway by pressing or releasing a modifier button. In various positions, this will change the transition and/or accelerate the transition. There are others of these, especially from guard, that seem to accelerate transitions, but just a little. I have not added those to the list, only Transition Switches with extreme acceleration. The videos for these have time code, so you can see for yourself.
This list is incomplete, I assume, please add any new Transition Switches you find to the comments and I will add it to the original list.
Full Guard Top (Not Postured) to Stacked Guard - Wrestling Transition level 2:
Press L1 and Transition up to Get Up, release L1 midway, transition is accelerated and becomes Stacked Guard making Full Guard Postured Up position unnecessary to achieve Stacked Guard.
https://www.youtube.com/watch?v=7ALyGgRAjmU
Side Control Top to Side Saddle Top - Wrestling Transition level 3:
Transition to North South, press L1 midway, transition becomes Side Saddle, a transition not otherwise available.
https://www.youtube.com/watch?v=NIzO7YvCiWQ
Full Guard Bottom to Half Guard Top - BJJ Sweep level 1:
Transition to the side to Get Up, press L1 midway, transition becomes sweep to Half Guard Top, a transition not otherwise available. Works well with the Full Guard momentum transition.
https://www.youtube.com/watch?v=JyVPXk7smig
Full Guard Bottom to Rubber Guard - BJJ Transition level 2 with Rubber Guard:
Press L2 and Transition to the side to Omoplata, release L2 midway, transition becomes Rubber Guard and is accelerated. A stamina advantage can speed this up drastically.
https://www.youtube.com/watch?v=sszGbKJ_uLg
Side Control Bottom to Sprawl Bottom - Must have Single Leg Takedown from here:
Press L1 and transition to the side to Single Leg, release L1 midway, transition becomes Sprawl and is accelerated.
https://www.youtube.com/watch?v=YAFCizg-PeU
Side Control Bottom to Half Guard Bottom - Must have D'arce Choke:
Press L2 and transition to D'arce Choke, release L2 midway, transition becomes Half Guard and is accelerated.
https://www.youtube.com/watch?v=lg5b3wcY4pU