I saw a old Nova episode on CIMAvax but I never saw any explanation of how it works (maybe I missed it). A quick search turned up this video, which provided a plausible mechanism.
The mechanism is to trigger an immune response against Epidermal Growth Factor which can act to spur some forms of cancer.
Unfortunately, it doesn't have a great success rate, but the recovery rate may be improved when combined with other strategies.
Serious major kudos to the Cuban scientists for this.