The first time I saw pavel the ketel bell russian dude perform these one-arm one-leg pushups I was very impressed. It’s taken a while but I’ve managed to get them.
The progression? Start with one arm pushups on incline, as high as you need to do them right. Over many months reduce the incline. When you can do about 15 on a flat surface, you’re probably ready to try the one-arm one-leg version. Without the second leg to stabilize, your body compensates with core strength and stabilizer muscles (intercostals etc).