The gradient you hold when setting the climbing track. 12 to 15 degrees is the efficient corridor; past about 20 degrees even good skins noticeably lose grip.
Skinning means gliding the boards over the snow instead of lifting them the way you would when hiking — over several hours that saves noticeable energy in your legs. The typical mistake is a track that's too steep: it feels faster, but it costs grip and rhythm, and you slip back a little with every step. Hold the gradient as constant as you can at 12 to 15 degrees, and beyond that set a zigzag line with kick turns rather than stubbornly stomping straight up. Flip the heel lifters up once the gradient turns moderate, not before — they relieve calf and achilles, exactly the muscles that tire first. What often gets missed: past roughly 25 degrees pure skinning becomes inefficient, and ski crampons or bootpacking take over. And an efficient track is not automatically a safe one.