Seated skids have a lot to do with a few forces in play.
1. Leg strength
2. Gear inches
3. Speed

If you are weak and have larger gear ratio going fast it will be much harder to resist the pedal rotation allowing you to stay seated. It just takes practice to figure out your timing. Seated skids are like skipping but without standing. If you really want to increase the stopping power while seated press down even harder backwards with your rear foot, pull up with front. This causes the rear tire to rotate backwards while still going forward which equals increased drag. Seated skids are like trackstanding, they come with time on the bike. Alot of its just technique.