Valentin, as far as the specific flexibility, I do not do anything special for the Undersomersault that is not normal to ordinary warm-up and daily flexibility requirements.
In the case of conditioning I will start using more of a fold in the body when the gymnasts are able to handle the strain on their lower back during the compression phase of the skill. Therefore, I do a lot of core training - backarches, situps etc. to strengthen the mid body and ensure that when the body is ready we can start utilising a deeper fold in the body and transition to eventually be able to perform the Undersomi's with a Stalder type of action. The boys in the video are not ready for this phase yet.