As Scott and bubbles stated, the adapters take a single 120 volt leg (L1) and share/jump it over to the L2 side. You are limited to the TOTAL amps of the outlet you are plugged into.
Instead of having 50 amps per leg for a total of 100 amps on your normal service...you are now sharing 30 amps between the two legs for a total of 30 amps (using the 30 amp adapter)...or sharing 15 amps between the two legs for a total of 15 amps (using the 15 amp adapter)
This diagram may help you see it, as the one on the right is the 50 amp adapter to 30 amp. You can see how the wiring works. Hope it helps