Learn the kip on the parallel bars (drop kip, glide kip): inverted-hang swing, explosive hip opening, spotting inside the rails and mistakes that cost strength.
The kip on the parallel bars brings you from a hang or inverted hang back into support on the rails with an explosive hip opening. The Swiss federation calls one variant the "drop kip": at the end of the forward swing you close the leg-trunk angle, push the shoulders back and lower into the inverted hang, swing forward and back, open the hip explosively at the end of the backward swing and thrust the legs back-up while the straight arms press on the rails, as in a backward swing-up [2].
The honest answer to "How hard is it?": The kip rarely fails because of strength but because of timing. BASPO lists the main mistakes exactly like this: the kipping movement is too flat or too steep (the opening goes in the wrong direction), and it takes a lot of strength because the timing is off and the unloading at the end of the backward swing is not used [2].
A competition classification: the FIG men's rules say that kips on the rings should be initiated from a deep fold with a clear, dynamic swing. Whoever does them with little fold and a lot of strength is deducted [5]. That is another apparatus (TRANSFER), but the same idea: the kip is swing, not a pull-up.
With help, lower from the L-sit on the parallel bars into the inverted hang position. The spotters support through under the rails at back and thighs [2]. Milestone: three lowerings, arms stay straight, control in the end position.
From a light swing you take the support position, close the leg-trunk angle, lower to the inverted hang and swing in that position, with and without help [2]. Milestone: five calm swings in the inverted hang.
This is the core. You swing rhythmically in the inverted hang and open the body slightly at the end of the backward swing. That is how you use the unloading [2]. Milestone: five swings with a small opening, always at the same point.
The kipping movement has a direction: back-up, not just up. Practise the correct kipping direction from the inverted-hang position on the floor [2]. Milestone: ten openings on the floor, the legs land on a marked line.
From the swing you open the hip explosively and thrust the legs back-up. The spotter gives an acoustic cue as the impulse for the kip thrust and accompanies back and hips [2]. In the glide kip the spotter additionally brings the legs over the rails so the pelvis does not sink, and lets go only when you are secure in the swing-up [1]. Milestone: three kips with help, shoulders over the hands.
Constant help does not suit learning success because you get used to it and fix mistakes. That is what the Swiss material says about the glide kip on the high bar [4] (TRANSFER to the parallel bars). The help should gradually become a safeguard. Milestone: two kips with safeguard only.
Make swinging in the inverted hang an exercise of its own before you kip: ten swings, each with a small opening at the end of the backward swing, as if you were kicking a shuttlecock with your feet. BASPO builds the kip exactly on this rhythmic opening [2]. Whoever has the rhythm needs less strength for the kip.
In the drop kip you lower from support to the inverted hang, swing and kip back into support [2]. In the glide kip you come from the forward swing, the legs go maximally forward, you close into the kip position and push up into support [1]. In the upper-arm kip you close the body maximally into the kip position, open to the candle position and support yourself again [1].
No, the kip is not a pull exercise. The unloading at the end of the backward swing and the explosive hip opening make the movement [2]. You still need support strength [6].
That depends on support strength and timing. Many need a few weeks with help. That is an experience-based estimate, not a study figure.
Yes. The spotter stands inside the rails and works from below [1]. Later the help is reduced to a safeguard [4].
Mostly it is the timing: opened too flat, too steep or too late [2]. Practise the swing in the inverted hang and the direction on the floor [2].