Gas enters through valves between the annulus and the tubing. Continuous lift injects all the time and suits wells that can sustain a steady rate. Intermittent lift injects in slugs and suits weak wells that would otherwise fall below a stable rate. Unloading valves start the well by kicking off the heavy fluid; the operating valve is the one that injects at the design depth and rate.
The injected gas raises the producing gas-oil ratio on purpose. A lift design that injects too deep, or too much, spends compression for little extra oil. A design that injects too shallow leaves a heavy column below the valve. The packer keeps the lift gas from bypassing the valves.