(As has been pointed out by MattM, my earlier comment is probably better placed as answer. I have added some more detail below).
Others may disagree, but I don't think this forum is the right place to learn the necessary skills.
To have any chance at all at coping with such a situation - against a stronger, heavier and determined opponent on top of you (and therefore aided by gravity) - you will require in-person training and lots of highly specific practice. Even then, the chances of coming out unscathed aren't great.
I don't in any way mean to discourage you from learning to defend yourself. There are many disciplines which can enable you to more confidently navigate the world if you're concerned about physical attack.
Explore Brazilian Jiujitsu and Krav Maga and perhaps Muay Thai or Mixed Martial Arts (MMA). Both have utility in regards to the situation you describe, and in regards to many other, far more likely scenarios, as well as providing many other benefits such as fitness, self-confidence and friendship.
Brazilian Jiujitsu equips you with techniques that utilise principles of leverage to enable you to unbalance an opponent in circumstances akin to the one in the clip you provided, and aims to provide practitioners with the ability to overcome stronger and heavier opponents.
Krav Maga emphasises 'street relevant' skills, including techniques designed to combat armed opponents.
Regular Muay Thai/Kickboxing/MMA training will equip you with a wide range of functional strikes and grappling techniques and a high degree of fitness. I mention these because, whilst they aren't strictly self-defence disciplines, they tend to be more widespread than Krav Maga schools.
Even a generic 'self-defence' class by a competent teacher would be very beneficial.
If you can't afford lessons or can't access training facilities for any reason, Youtube contains some excellent videos that may be of some use if you practice the techniques. There is also a lot of junk out there. With this in mind, this playlist isn't a bad introduction to Krav Maga. Some other Stack Exchange Members may be able to provide some better links.
Your best defence will always be avoidance. You should of course never find yourself in such a situation. I understand your question might stem from curiosity alone, but if you're at risk for any reason, seek police advice as soon as possible.