A person sitting on a swing and swinging, suddenly stands up when the swing passes through its lowest point. 

Angular momentum of the system is conserved during standing about the axis of swinging.

As the center of gravity moves towards the hinged point, using $\mathrm{L}=\mathrm{mrv}$ we can say speed increases.

Also, the gravitational potential energy also increases.

Since, work is done by the person to stand, that explains the increase in total mechanical energy.