Pulling Up vs Snapping Back:
There is a fine line between 'pulling up' and 'snapping back' the knees, and it is a difficult area to control for many individuals. Hypermobile individuals must focus on lengthening the leg, rather than overly contracting the quadricep to 'pull up' when straightening the leg, to prevent the knees from going too far back. Many dancers feel that they have to bend their knees, however, this is not the case. You do not have to bend the knee, just make sure that it is not 120% straight! This is also very important when walking normally, not just in ballet.
In Class:
Avoid the temptation to take your heels too far apart to create space for the knees. A space of 3cm (1 inch) should be sufficient for all dancers to develop control of the legs while maintaining a nice line. Under this, you may feel the kneed to bend the knees, more than this, you will increase the development of pressure through the front of the knee. Keep in mind that hyperextended knees also make it harder to fully rise onto Demi-pointe!