Importance to Goalkeepers
Pass backs allow players to relieve pressure by passing it back to their goalkeeper. This also allows the goalkeeper's team to gain an additional player for possessing the ball and pushing back up the field.
Pass Back Rules
The main rule here is that when a teammate passes the ball back to their goalkeeper or is in control of the ball at their feet, the goalkeeper cannot touch or pick the ball up with their hands. Some exceptions to this rule are (when the goalkeeper is allowed to pick it up:
If it deflects off of another player before the goalkeeper picks it up
If the ball is unintentionally knocked back to the goalkeeper
If a player on their team heads or chests the ball back to their goalkeeper
Goalkeepers must always use discretion in these situations, because a ref could view intentionality and control differently than the goalkeeper. If the ref calls a pass back (the goalkeeper uses their hands), the other team receives an indirect free-kick where the goalkeeper used their hands. This is the same result as when a goalkeeper uses their hands outside of their 18-yard box. In both cases, if the goalkeeper has had previous warnings or stops a good goal scoring opportunity, they may also be given a yellow or red card.
Pass Back Strategy
Since pass backs allow a goalkeeper to join the play as a player, the best way to improve these skills (first touch, passing, communication, positioning, etc.) is by joining teammates in drills and scrimmages as a player. The ideal practice would be a game with small nets and no goalkeepers (passing through cones to score for example) where the goalkeeper plays as a central defender. This will improve the goalkeeper's skills and increase their involvement in the play, but focus on distribution and positioning that they will use in the game. This will also help a goalkeeper in winning battles for the ball outside of their box.
In terms of the actual technique and strategy of pass backs, there are a few things to keep in mind: decision-making, communication, and positioning.
For decision-making you have to remember that there is usually no one defending behind you, so you cannot take the same risks as your teammates. If you are facing significant pressure, it is important to move the ball quickly (with fewer touches). If you have more time you can often wait if your team is ahead to waste time, give a short pass to a teammate to build slowly out of your zone, or scan the field looking for a deeper option.
Communication is important, because it allows the goalkeeper to let their defender know if they have pressure, whether the goalkeeper is in position to receive a pass back, and where the goalkeeper is. This is also crucial in situations where your defender is looking to head the ball back to their goalkeeper. To highlight the damage of miscommunicating I'll share how I scored my only goal as a goalkeeper. I kicked the ball deep into the other teams half and after it bounced once, the defender tried to head it back to his goalkeeper (this is a very common and effective play). However, the goalkeeper had come out to play the ball himself and neither of them communicated with the other and the ball was headed over the goalkeeper and into their own net. Miscommunication happens all the time in sports, but the repercussions are often much worse when the goalkeeper is involved.
Positioning is also an important way to reduce risk. The best way to open up for a pass back is to shift so there is a clear lane from your teammate to you where it will be difficult for an opposing player to get to. It's also helpful to be well in front of your goal-line and beside your net whenever possible. This limits the danger if the goalkeeper misplays the ball or is under high pressure, because the ball is further from the net. If the goalkeeper kicks the ball and it is blocked by the opposing player, it's very likely to end up in the net if the goalkeeper is close to it.
In the following diagrams I'll show what was probably the most common situation for a pass back and the positioning and decision-making that I use in it most often. The goalkeeper is shown in yellow, teammates in orange, opposition in red, and the ball in white. Arrows show movement. One common strategy that you'll notice is for the goalkeeper to take a touch away from pressure in order to increase time and space for them to make a good pass or long kick.
In the situation above, your teammate has the ball and is being pressured by the opposition. The goalkeeper comes to the side of their net and calls for a pass back. Due to the minimal pressure the goal keeper can take a touch to the inside and send a long ball up the field or find a teammate with space (orange player on the left). If the opposition only pressures slightly, I'll still have time to shift to the middle and find an option. If my teammates don't drop, then there are likely plenty of targets up-field for a long kick.
Now this second situation is similar, except this time the opposing player anticipated the pass and applies heavy pressure to the goalkeepers left. In this case the goalkeeper is best suited to take a touch to the outside (away from pressure) allowing them time to find an option for a long kick up the side or for a central midfielder (teammate with the arrow) time to shift down and be an option to split the pressure and turn up-field.