Powered dispencers are refreshed and shoot twice if you open and than close any container.

Steps(in my case):
1)set clock-generator (double hoppers witone item or double 0.4 repeaters) with output on dispencer
2) place container somewhere nearby (for example, 4 blocks away - not necessary to be close to dispencer)
3)Load something into disp (im my case - minecarts - dispencer facing rails)
4)Open container (chest, hoper, etc.)
5)Close container and listen to the sound

Result: dispencer clicks twice and shoot 2 items (minecarts)