take this as you will @Nate_Pearson but if were talking on a trainer id really suggest seated sprints. this literally has nothing to do with the whole rocking a bike/breaking frames/stability etc issue. i personally think learning to do hard seated sprints will force you to learn to hold full body tension better then being able to flail out of the saddle like most people are prone to doing ESPECIALLY when youre fatigued and tired at the end of a race. if you can practice turning over a big gear and accelerating it from a slow speed/stop even for a 10 second effort youll learn to hold tension that should transfer over to out of the saddle outdoor rides. give it a shot and lemme know what you think.
my personal favorite seated sprint workout is find a hill(really just a bump) that you can roll into slowly in a relatively big gear and just sprint over it as hard as possible. ideally taking 10-15 seconds till you get to the top, then just roll around for 5-8minutes and do it again. repeat untill your power starts to drop excessively.
indoor seated stuff id have to go with 10 second big gear seated accelerations. find a gear that you can pedal at 60rpm and then sprint so youre in the 110-120rpm range or there abouts for a peak. 3-5min recovery. repeat. i do 3 starting with my right leg and 3 starting with my left. then i repeat the entire block but in a small gear where im hitting 200+rpm. one block gets the strength gains, one block works on the ability to contract muscles exceptionally quickly! then go take a nap, if you do it properly youll just lay down next to the bike afterwards