The clear sends the shuttle high and deep into your opponent's back court. Learn the right contact point, use your body and avoid arm-only swings.
The honest answer: The clear is a high overhead stroke that sends the shuttle deep into your opponent's back court and lets it drop almost vertically there. It buys you time when you are under pressure and forces your opponent backwards. For most players the real game starts with it.
Two things make the clear hard. First: it needs technique, not strength. If you hit only with the arm, you hit short and flat. Second: you must be properly under the shuttle beforehand. A study with 29 players (14 skilled, 15 novices) showed that the position of the body relative to the shuttle directly influences launch angle and height of the stroke [2]. The study examined the smash, but the conclusion holds for all overhead strokes: position before stroke.
An honest remark: there is less research on the clear itself than on the smash. Where I draw on smash studies, it is stated. Everything without [n] is coaching practice.
Turn sideways to the net: racket shoulder back, the other shoulder pointing to the net. The free arm points up at the shuttle. This helps balance and aiming. Important: run under the shuttle first, then hit. The contact point is above you and slightly in front. In the study, the standing position was the key to this [2].
Practice tip: For beginners it worked well to stand comfortably under the hanging shuttle and then step one foot back [2].
The elbow goes up, the racket hangs relaxed behind the head, almost like a throw. The grip stays loose. Skilled players relaxed triceps and wrist flexor right after contact, unskilled players did not [5].
Now you rotate hips and shoulders. In the smash, a study showed that skilled players use clearly more trunk rotation than beginners and thereby achieve higher shuttle speed [3]. This is a transfer to the clear: whoever hits from the trunk needs less arm. Then extend all the way up.
Hit the shuttle at the highest point of your reach, slightly in front of the racket shoulder. The forearm rotates inwards, the wrist snaps through at the end. Do not just push the shuttle away, hit through it.
Milestone: Seven of ten shuttles fly higher than the net post and land in the back half of the court.
Right after the stroke you run back into the ready position and make the next split step. Whoever stands still after the stroke gives away the advantage the clear gives.
Do the "towel test": place two towels at the baseline, one left, one right. Your aim is to hit both alternately. This trains direction and length at once, and you notice whether you catch the shuttle too early or too late. Counting beats feel: note ten attempts per session.
Run sideways under the shuttle, take the racket behind your head, rotate trunk and shoulders and hit the shuttle at the highest point slightly in front of the racket shoulder. Hit through and immediately run back to the centre. The position under the shuttle determines angle and height [2].
Usually you hit too late or only with the arm. Run further back so the shuttle stays in front of you, and use trunk rotation; skilled players rotate the trunk clearly more than beginners [3]. A grip that is too tight also slows the racket.
The clear flies high and deep into the back court, the drop falls just behind the net. Both are started from the same preparation, only at contact the stroke becomes soft instead of hard. Learn the clear securely first; the drop builds on it.
When you are under pressure or need time. A high, deep shuttle buys you a few seconds to get to the centre. It also forces the opponent backwards. This is coaching practice.
Badminton Lunge: Reaching the Net Safely and Getting Back