Ukigatame (knee-on-belly) is probably the most important hold-down taught in judo. It is one of the two most-common transitions from standing to groundwork (the other is makikomi). I recommend all of my students when practicing nagekomi of a throw that does not involve makikomi, always finish in ukigatame (kneeling on top of a downed opponent) if possible.
Here are a handful of things that you probably ought to be able to do from ukigatame -
- get completely away from uke with minimal exposure to being grabbed or struck
- maintain ukigatame as uke tries to bucks, shrimps, bridges, and rolls
- transition to tateshiho, munegatame, or kesagatame
- attack simple chokes, like katatejime, in order to expose uke to armbar attacks
- attack jujigatame or hizagatame on the near or the far arm falling backward or forward
If anything on that list seems like a stretch to you, I would recommend doing some drilling on that skill with a non-resistant partner, then try it out in randori.