The basic game of match race sailing consists of two yachts, each with the objective of crossing the finishing line before her opponent while complying with the racing rules (and sailing instructions, etc.).
As with the rules of other sports, the yacht racing rules are generally framed to give advantage to whoever is in the lead. Each yacht's objective is simple: to be the first to cross the finishing line with no outstanding penalties, having started correctly, sailed the course, and complied with any special requirements. The simplest way to achieve this is to be in an advantageous, controlling position at the start and to remain ahead and in control for the entire race. With evenly matched boats and crews, a good start usually results in winning the match, although the new tendency for downwind finishes means that a small lead is not always sufficient to ensure victory.
In the pre-start period there is no 'proper course', which is significant from the point of view of the rules. The two yachts manoeuvre against each other with two objecitves: to encourage the other to infringe a rule so that she will have to take a penalty after the starting signal, but, if that is not achieved, to be in a controlling position at the start or to get a significant advantage at the start. Being 'in control' at the start doesn't necessarily mean being in the lead at the moment of the starting signal; the object is to be 'in control' soon after the start. With superior speed and timing it is possible to be second across the starting line, but achieve a controlling position very soon after. Depending upon the experience and confidence of the skippers and crews, there is often a psychological advantage to be gained by the skipper who is clearly in control during the pre-start period.
During the race on upwind legs the yacht in control is either ahead and in such a position that any windshift will not benefit the other yacht, or close ahead in a position where her dirty wind or backwind is adversely affecting her opponent, or her opponent is trapped and unable to tack because of the proximity of the controlling yacht.
On downwind legs, protecting a lead is often difficult and, unless well ahead, the leader will be concerned with ensuring he is inside or ahead at the next mark rather than being 'in control'.