This is a loaded question Backswing! But I’ll do my best to answer.
1) It is correct that a slight downward strike (what we call a negative Angle of Attack in the teaching world) is desirable anytime the golf ball is resting on the ground. This slight downward strike (in the range of 2-4 degrees downward) helps create solid contact in the center of the club face.
2) The lead wrist is a contributor in creating this negative attack angle, but so are other moves with the body and club. More often than not, if your lead wrist is breaking down before impact, it’s usually the result of poor body mechanics. Finding a good coach or PGA Professional can help you identify if this is the case.
3) While “supination” is actually rotation of the lead forearm counterclockwise (for a RH player), it can help create more flexion (aka: “bowing”) of the lead wrist. Together, these moves help square the club face and assist a player to hit downward slightly.
4) Here’s the catch: How much of this flexion (bowing) move a specific player requires depends on a few factors; a players’ lead hand grip being the most important. A stronger grip will require LESS flexion at impact to square the face and hit down, while a weaker grip will require MORE flexion of the lead wrist to square the face and hit down effectively. Again, working with a trusted coach to help you identify your grip and how much flexion you need is recommended.
ADVICE: Once you have wrapped your brain around how your lead wrist needs to flex and how much, the real secret to the sauce is actually being able to flex it the desired amount in a real golf swing at full speed. The biggest mistake I see golfers make is trying to flex the wrist at or thru impact. While the idea seems logical, the reality is by that point in the swing it is too late to try and implement the move. The club swings the fastest thru impact; trying to control it through wrist mechanics at this point is basically impossible.
I recommend the lead wrist flexion to happen much earlier, preferably in the transition-area of your golf swing, at the latest. Therefore, my advice is to get your ducks in a row early on so you can deliver the club at impact with better wrist conditions much more naturally and effectively at full speed.
DRILL: Get in front of a mirror and practice flexing your lead wrist the desired amount in early transition, the point in your swing when you change direction from backswing to downswing. Feeling a flexed lead wrist at this point helps simplify the rest of the downswing allowing you to deliver a square face and downward strike more often. After working in a mirror, go ahead and hit balls performing the same drill. I recommend pausing at the top of your swing to feel your wrist then swinging through at 50% effort to train the new move. Working up to full swings and more speed over time is the goal.