Start on a climb without the front wheel lifting or the rear wheel spinning: bike across the trail, low upper body, uphill foot down. 5 steps with milestones.
The honest answer first: on a steep slope you pull away by placing the bike across the trail, sitting on the nose of the saddle, bringing your upper body low toward the bars and pushing off with your uphill foot while pedalling at the same time [1]. It is not about strength, it is about sequence.
DIMB lists "starting on a climb" as its own learning area in the basic module of its riding-technique training, next to base position, brake modulation and climbing techniques [2]. That shows it is foundational knowledge, not something for show-offs. And it matters for safety. If the rear wheel spins or the front wheel lifts, a fall backwards is possible, which was rare, though, in a video analysis of 534 crashes (3.2 percent) [3].
How hard is it? You learn the movement in an hour. On loose or wet ground it still feels tricky later.
Starting on a climb is 30 percent technique and 70 percent the decision to put your foot down only when the bike has stopped.
If the trail is wide enough, turn the bike diagonal to across. This reduces the effective gradient [1].
Milestone: you can hold the bike safely on the slope with the brake applied.
Your uphill foot stays on the ground and you sit on the nose of the saddle. The downhill foot is on the pedal, crank at about the 1 o'clock position [1].
Milestone: you stand so that you do not roll back without help.
Bring your upper body low toward the bars. This prevents the front wheel from lifting [1]. On a steep climb you pull the bars backward and down, not upward [4].
Milestone: your nose stays near the bars while starting and the front wheel stays down.
You push off with the uphill foot and pedal at the same moment [1]. This gives the rear wheel drive right away.
Milestone: five clean starts in a row without putting a foot down.
The uphill foot goes onto the pedal immediately and you keep pedalling with a round, even stroke. Clip in later if needed [1] [4]. Eyes uphill, steer the bike uphill.
Milestone: after the start you ride another ten metres up the slope without wobbling.
When starting on a slope the bike can roll backwards or tip over backwards. Backward falls were rare in the video analysis at 3.2 percent [3], but the most common injury sites in mountain biking are still the head and upper extremity [5]. In a meta-analysis across cyclists, a helmet cut serious head injuries by about 60 percent [6] (cycling in general, not only MTB).
Hold the brake before the start and choose a slope without a drop-off. Never practise at an exposed spot. This guide explains and motivates, it does not replace supervised training.
Clip in while you are rolling, not while standing. BASPO explicitly says to put the uphill foot onto the pedal immediately and keep pedalling, you can clip in later [1]. That protects you from the most common mistake: fiddling with the pedal while stationary as the bike rolls away.
And if the start fails twice in a row? Find a flatter slope. The learning effect comes from starts you manage about two times out of three, not from starts that always fail (practice). Increase the steepness in small steps, not in jumps, and push the bike without guilt when a slope is beyond your form on the day.
One more note on the rear wheel: on loose ground it spins more easily because there is less friction (practice). Then it helps to move the start to a firmer strip or choose an even easier gear.
Place the bike across the trail, uphill foot on the ground, sit on the nose of the saddle, downhill pedal at about 1 o'clock, upper body low, push off with the foot and pedal at the same time. Then put the foot on the pedal immediately and keep pedalling [1].
Your centre of mass is too far back or you pull the bars jerkily. Bring your upper body low toward the bars, which prevents the front wheel from lifting [1].
Seated, on the nose of the saddle. Out of the saddle the rear wheel lacks pressure and spins [4].
No. DIMB even recommends flat pedals for learning [2]. With clipless pedals you clip the foot in only after the start [1].
Rolling Over Obstacles on a Mountain Bike: Taking Roots and Rocks Smoothly