Emphasis on the up stroke

Compared to sitting in the saddle, the big advantage of the standing position is that you can emphasise the up stroke (back of the ‘square’), making it more active at both the knee and the hip, as the more open angle at the leg allows the hamstrings more room to pull, and at the hip assists the hip flexors in getting more involved.

Where on the flat the effectiveness of the upstroke is limited somewhat by the shortened position these muscles are in, this is not the case when standing, and the upstroke is more able to share the workload, almost matching the down stroke if the challenge is significant enough (such as on a very steep gradient).

It can be said that you are literally pulling yourself up the drag or gradient, rather than just pushing into it (which is more the feel of the effort when seated). The lateral movement of the bike created by the upper body together with this increased effort on the upstroke is what generates more force.