The hit puts speed into the disc: front foot, hip and trunk rotation, late acceleration. What the evidence shows and what not, plus mistakes and protection.
The honest answer: the "hit" is the moment when energy from legs, hips and trunk passes into the throwing arm and the disc. I must admit straight away: the term comes from the player scene, and none of the sources I read defines "hit" or "plant-foot brace" in exactly that way. What the sources give is a description of the movement: a club handout describes that the front foot is placed a little forward and then loaded, upper arm and elbow are pulled forward and the trunk rotates as vertically as possible around the spine [1].
Why the effort? A study on flying-disc forehand throws shows: throwing distance was related to the disc's spin speed (r = 0.72 in 10 skilled, r = 0.79 in 11 unskilled throwers), and the increase in spin speed was related to the angular impulse of torque; the authors suggest that unskilled throwers could build a larger torque, as skilled ones do [2]. That is a TRANSFER: it was about forehand throws with flying discs, not the backhand drive in disc golf.
How hard is it? This is the pro level: the movement is hard to feel from the inside, and speed at the cost of control gets you nowhere. There is no timeline.
Before you combine run-up and rotation, practise the standstill throw. Both feet stand on a line in the throwing direction, the throwing shoulder points in the throwing direction, the arm is loose [1].
Milestone: Ten standstill throws, the disc flies straight and spins calmly.
The disc goes straight back, trunk, head and hips turn along, the rear foot is loaded [1]. If the reachback is messy, the hit will be too.
The front foot is placed a little forward and then loaded [1]. The weight moves forward with a leg push, as the handout describes for putting (practical transfer to the drive).
Milestone: You feel the weight move from the rear to the front foot without falling forward.
Upper arm and elbow are pulled forward, the trunk rotates as vertically as possible around the spine [1]. The disc is accelerated forward on a straight line, relatively close to the body [1].
Maximum acceleration comes only shortly before release, with forearm and wrist [1]. The trunk stays relatively upright and does not bend in the throwing direction [1].
Milestone: In the side video the trunk is upright at release.
The throwing arm swings through loosely, ideally back in the opposite direction, i.e., not deliberately stopped; only now do you watch the disc [1]. At the end the throwing arm does not point upward and stays as much as possible in one plane [1].
Don't make the hit a feat of strength but a sequence: feet, hips, trunk, arm, wrist. The handout describes exactly this sequence with the acceleration right at the end [1]. If your arm works earlier than your hips, you lose speed and load the shoulder.
In scene terms, the moment when trunk and arm swing come together and the disc is accelerated. I have not read a source that defines the term; the Graz handout does describe the sequence: foot forward, trunk rotates, acceleration shortly before release [1].
In a study with flying-disc forehand throws, throwing distance was related to the disc's spin speed [2]. That is a transfer finding; I have not read a study on the backhand in disc golf.
Relatively upright, not bent in the throwing direction [1].
Yes. At the moment of release at least one supporting point must be in the teeing area, and all supporting points must be within it [3]. Before and after you may stand outside.
Jump Putt in Disc Golf: Rules, Risk and Clean Execution