I wouldn't cut it back if I could help it. Sawzalls work great for dividing bamboo (just do your best to avoid new culms). The best time of year would be right now - the growing season, during the in spring and early summer.

The plants underground system of roots and rhizomes provides opportunity to divide off one or more culms to make new plants. This technique is more likely to be successful when the parent plant is healthy and pest free and appropriate care of the new division is taken.

Often cutting off a single culm with a healthy budding rhizome will be enough to result in a new clump of bamboo but you will achieve a much better plant by removing more than one culm that are joined at the base. You will have provided a much better chance for success for generating the new plant with two or more culms.

Rhizome shoots are connected to the root ball underground, are very hardy, and can be pretty tough to cut. Use a sharp spade, secateurs, or some loppers for this job. Try to cut it off quite close to the root ball so you leave a tidy cut.

Dig the young rhizome shoot out carefully while watching that you don't damage the roots. Once you have them out of the ground it is important not to let them dry out, I usually put them in a bucket of water temporarily.

Please be careful not to damage the parent plant, particularly if you are taking the division from a relatively young clump. It is better to divide from a well established plant.

While you are working on your bamboo propagation do not let any part of the plant dry out. Ensure you keep it moist during the operation, while transporting, transplanting, and during storage.

It is best if you have already prepared your ground or pot ready to transplant the young plant to. If your new clump doesn't quite fit into the hole or container trim it down a bit with secateurs.

Once planted be sure to water it in well.

You can also shorten the top growth by up to half which will help reduce transpiration while the new plant gets established in its new home.