There are many ways to improve your B-side. Here’s a simple and effective one.
How? Lie with your back on the floor, bend your knees and place your feet on the floor. Now lift your hips as much as possible, squeezing your buttocks, then go back to the starting position. With a rather slow and controlled movement, move your hips up and down in a succession of contractions (up) and releases (down).
Alternate one minute of movement to one minute of stay at the peak. Do this for 4 to 8 minutes, depending on your level of training, possibly without pausing.
Variation: it is possible to do the whole exercise holding an object (i.e. ball, book, pillow) between your knees, pushing the latter against each other in abduction.
To increase the intensity of the exercise further, you can put a weight on your hips: ideally, a heavy disk of 6, 8 or 10 kg, depending on the desired intensity.