Bum-off-seat cycling Is their a particular term for "standing" while cycling, and how is it done?  I've never been able to manage it, nor for that matter, flinging my leg over the saddle while the bike is still in motion, when the ride comes to an end.
It can be called "standing sprint" if used by a road race bicyclist during a race/training. At MTB, it can be associated with "attack position", where one leaves the saddle momentarily (or for quite prolonged periods of time in fact, thanks @mattnz) to not be thrown out of the bike by the saddle launching the rider into space.

For regular commuters, Sheldon Brown considers the need to stand up while pedaling as a sign of a bad technique or poor bike fit:


  If you find yourself standing to accelerate, on level ground, it is a sign that your gear is too high or that your saddle is too low.


Also from there:


  Standing pedaling doesn't make you any faster, except in the very short run. 


So, check your gearing technique and bike fit.