The big (thumb) lever on each trigger shifter, upshifts, against derailleur spring pressure, that's why it's the big lever, for more leverage. The small (index/trigger finger) lever on each trigger shifter, downshifts, with the derailleur spring helping. (I think) There is also a spring inside the shifter, to make the mechanism downshift when triggered, even if no cable tension; if that doesn't happen, you may need a new shifter. Also be aware, each trigger only shifts one cog or chainring at a time. So to downshift from 3 to 1 on a left triple shifter, you need to pull *and release* the small trigger lever twice, 3 to 2, then 2 to 1. This is known as ratchet shifting, some cars are set up that way, World Rally cars were set up that way with a mechanical ratchet shifter, one yank per shift, until electronic paddle shifters on the steering wheel superseded them. Motorcycles (without automatic transmissions) have been ratchet shifted for over a hundred years.