The defensive lines of Futsal are associated with the idea of space, as if there were imaginary lines on the court, where athletes are positioned to organize the defense. The higher the defensive line, the less space is left for the opposing team to play, however, the higher it is, the more vulnerable your defensive system will be. These imaginary lines are marked from attack all the way to defense. The total number of lines will change according to where you are from and/or your beliefs. I'm from Brazil and was thought that there were 5 lines in total, 1 being the highest on the field. So this video explanation will be based on that but the concept is generally the same.
The first line of defense or line 1, is on the opponent's defense court, next to the PK spot, the so called pressure marking. This line is set that high, leaving no or little space for the opponent to play. This type of marking can be used at the end of a game in which the team is losing and wants to turn over the score. It is a very dangerous tactic, as it involves a great risk to the defense.
The second defensive line or line 2 is the half pressure mark. In this defensive set-up, the goalkeeper has a little more space to release the ball into play, however, as soon as the ball is dominated by an opposing player, the team pushes forward its line, increasing the pressure in marking.
The third defensive line or line 3 is the half court defense, because in this defensive position the team positions itself in the middle of the court, leaving opponents unmarked in their defensive court, however when the opponent enters the attacking court, they suffer strong defensive pressure. The purpose of this defensive positioning is to prevent the infiltration of the players and "balls in the back or through balls behind" as it decreases the space in the middle of the court, offering good counterattack opportunities.
The fourth defensive line or line 4 is a type of lower defensive positioning, set close to the 10 meter direct kick spot, on the defensive half. In this system the team waits in a compact way within its own half and, from that point and beyond, defenders will start fighting for the ball. Opponents will have lots of space to keep the ball, however, they will not have spaces for infiltrations or give and gos. The purpose of this defensive positioning is also for counterattacking opportunities.
The last defensive line or line 5 is a very low defensive position, set practically on the penalty mark on the defensive half. The so called, "Park the bus." The team stays very compact within their half court, blocking shooting opportunities and applying pressure to steal the ball and go on counterattacking situations. This method is applied in a game either when the opposing team has a much higher level than the other team, when the team is winning and cannot concede any goals on the last minutes of the match or when the opposing team is using a goalie 5th attacker, which is when the respective goalie on the opposite team steps onto the offense half to create numerical advantage.