Online videos can help you, if you already are a trained ballet dancer, to perform better and better. Keep in mind that ballet needs work and needs to be learned with a real teacher, and "on line lessons" will never avoid taking real ballet lessons, with a real teacher in a real school.
In any case, do not perform any work en pointe before you have been granted to dance en pointe by a professionnal ballet teacher. Otherwise, you may hurt yourself (in the worst case) or take very bad habits (in the best case), that may lean you to a poor en pointe performance.
Remember, before beeing an art, ballet is a very athletic sport, and as any sport, needs serious training.
Have a watch to this page about ballet learning ballet from YouTube ballet videos before performing any video work.
Stretch for a leg split in ballet by warming up the hamstrings and hips with all of the plies, and do a hamstring stretch starting with the knees on the floor and moving in to a split stretch. Stretch the hamstring forward and keep the hips warm and open
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