The Point in Line is a counteroffensive technique in all three weapons. Its purpose is to deny an attacker the ability to hit without first removing the threat the point presents. The Point in Line is established by extending the arm fully threatening an opponent's target with the point prior to the start of an opponent's attack, thereby gaining right of way in the right of way weapons and establishing a real threat in epee. Traditionally, the Point in Line is extended at shoulder height, in sabre with the guard rotated 90 degrees. From a fencing standpoint that position makes sense, as it gives you the greatest reach. Other locations for the extension risk the referee not considering the extension a Point in Line. Once a Point in Line is established the fencer may advance, retreat, or lunge maintaining the right of way.
The "prior to" part of this is important - a Point in Line extended into a developing attack is simply a stop thrust. Given the tendency of referees to regard any forward movement as the attack, this means either an extension must come out before the opponent starts a new series of footwork (a momentary pause in the action is the safest time), or the fencer must extend in preparation, which may mean that sufficient retreats are in order for the referee to realize that the attack is clearly one or more advance steps behind the Point.
An opponent can deal with the Point in Line by a feint which causes a nervous fencer to give up the line, by leverage, by percussion, or by simply outwaiting the fencer. Anything that causes the fencer to move the point from its threatening position invalidates the Point in Line.
The fencer can deal with attempts to remove the Point in Line by derobing to avoid the attempt. However, the derobe must be kept within the target - wide and you risk losing the right of way as the threat no longer exists. Against an attack that does not try to remove the blade, a step forward or lunge collapses the distance, removes the attackers options, eliminates lock out time as a factor, and results in a solid hit with a nice bend to the blade the referee can see.
Why worry about a bend? There is an oddment in the interpretation of the rules by referees - although references to point arrests have been eliminated from the sabre rules (the rules state clearly that any hit with the front, back, or flat of the blade is good, and in fact the point is not enumerated as a touching part of the sabre blade, see T70.1 and T70.2), the Fencing Officials Commission interprets the Point in Line as having to arrest with the point. So, if the referee chooses to believe that your hit grazes, rather than clearly arrests, he or she is at complete liberty to disregard your touch. This is silly, is not what the rules say, opens up an avenue for arbitrary favoritism by the referee, and unfairly disadvantages the fencer whose chest is away from the referee - but it is an unappealable matter of fact and fiat by the referees.