Bascule refers to the telescoping of the head/neck for the gait (this occurs in walk/canter, more so when the horse is in working gaits, less so in collection). The head and neck should not routinely go below the withers, it weights the forehand and slows the horse. Momentarily when completely free, but not sustained. The horse must figuratively and literally be up to the hand, then longitudinal flexion (bit acceptance) comes over time, with lateral flexibility.
Bending on a circle (shoulder fore=a step onto a 20m circle, shoulder in=a step onto a 10m circle) all help to fill out the outside rein, establish bit acceptance/flexion. But in walk there STILL must be bascule allowed within the walk. If not the head will start to nod, the flexion will be behind the atlas/axis (poll area), and the walk will slow and the horse will refuse the retaking of contact.
In trot, it is important that the horse stays with a mobile jaw and arced out to the hand and active (no closing of the throat latch).