There are several ways to strike the heavy underspin ball and they all can work well, it is a matter of what you want to do. You have fast, medium, and slow loop.
Slow loop is the safest and easiest to do. Every loop position, balance, and leverage is needed to make the necessary explosion to transfer energy to ball and overcome/continue spin. Key point to slow loop is position, allow ball to drop, even below table if needed and explode mostly up - more upwards vs heavier spin. You can keep a looser wrist at first, and as impact becomes better timed, you can firm up grip right at impact. Blade speed, angle, and vertical explosion are key. You cannot do any of these loops out of position or balance/leverage.
Medium loop you are impacting ball a little more direct, only a little more, you are opening blade a little more and swinging forward more. Best to take ball not too long after top of bounce.
Fast loop you have to open blade, but REALLY make explosive forward BANG into the ball explosion. Best to take the ball really early on rise while it still has vertical energy - this allows you to use a more forward swing for more speed component of power.
A lot of modern table tennis is built around taking the ball early and playing fast close to the table... but it isn't an absolute. Any shot sucks if it doesn't land. If you can make a slow heavy loop land 80%+ of the time in a match and trouble opponent, then it is an outstanding match shot and you have no need to play macho man table tennis... at least not immediately... although it would be a good strategic development goal to learn looping at all three speeds to better handle more situations and have more tactical options.