To increase human stage by 2, 10 sec after game start:
trigger_always: target = delay
target_delay: targetname = delay, target = stgctrl, gate = 262146, wait = 10
target_stgctrl: targetname = stgctrl
The gate value is sent to the target_stgctrl. 262146 = 262144 (which is TEAM_BIT = 1 (for humans)) + 2 (how much to change stage by, 0/1/2).
To change stage lower you need to add SIGN_BIT 1 in the incoming gate value, which is 131072, so to set human stage lower by 2, you would need gate value 262144 + 131072 + 2 = 393218.