Essentially, if you are perceived to present an increased risk to an insurer, you will pay more for cover or you may not be able to avail of certain forms of insurance. Underwriting is the process by which insurers determine the risk you present to them in offering you insurance cover.
All of the factors that affect the likelihood of you making a claim on your income protection policy may be subject to certain underwriting decisions. So your age, health, family health history, smoking status,and your job or hobbies can all be considered to present a normal level of risk or a higher level of risk to the insurer.
In terms of hobbies or activities you undertake, you may be accepted without providing further information, or you may be asked to provide the insurer with further information, in the form of a questionnaire. This will ask you for detailed and specific information on your hobby, how you engage in that hobby, whether you compete etc. This information will guide the insurer to an underwriting decision on your case.