Throwing a great pass can carve up a defense unlike any other facet of the game in rugby. A well-timed, well-placed ball puts a defense under enormous pressure and causes fractures in the defensive line. As long as the ball is placed accurately, on time, and in a position that the receiver can catch the ball on the move without breaking stride, it doesn't matter what it looks like. Heck, if we're looking at football, think of Peyton Manning's passes. One of the best quarterbacks in the history of the game rarely threw a spiral, but he did tick the boxes of accurate, on time, and in stride.
Put the ball out in front
First of all, he or she needs to be able to catch the ball in a position that makes it easiest to get the ball loaded into triple threat position (pass, kick, carry). If the receiver has to reach down or up or sideways to catch the pass, it will take them a while to reload into triple threat. In rugby there is very little time to work with before the defense is on you, so every millisecond is massive.
Secondly, the whole flow of the passing line works best when everyone is in sync. If the ball is thrown behind the receiver, the rest of the line gets thrown off because they all have to adjust for the bad pass so that they don't end up in front of the ball and in an offside position. This causes the line to slow down and lose momentum, as well as causing certain players to be moving at different speeds and the whole rhythm gets thrown off.
Thirdly, you want to put the ball out in front so that the receiver can hit on to it with some pace. For a defense, a flat-footed attack is far easier to defend than one that is coming at you with speed. When players attack with speed it forces the defender to respect the ball carrier, because if he or she doesn't the runner will blast right through. This squares up the defense and opens up holes across the line when defenders get sucked in.
Transfer the ball quickly
Rugby defenses can close space in an instant, so wasting time between catching and passing is something an attack needs to avoid when shifting the ball. Try to catch the ball as you would load it to pass, and see if you can avoid the "catch", "reload", "fire" method. Many of us will catch the pass, then dip the ball down or start swinging it to give it some momentum. This takes too long and will result in a hospital pass.
Try to catch the ball and then pass the ball from the same height with which you caught it, as if the ball were on one tight string. The amount of time you waste reloading the ball can be the difference between scoring in the corner and getting tackled out of bounds a meter short.
Stay straight and stay tall
This is probably the hardest part of passing, but it is something that can give you an invaluable skill on the field. Staying square on your pass and not "drifting" sideways is extremely important. By running sideways, you immediately make it much easier for the defender to defend you. With his or her momentum already going in that direction, your pass to the next guy or girl won't force the defender to change direction or momentum. They are effectively able to guard both of you.
Now, if you stay straight and square up the defender, he or she has to pick: Do I go with the pass and try and get to the receiver, or do I hold my ground and stay on the player with the ball? This is what you want as an attack -- to keep the defense guessing. If they suck in and stay on you, you'll shift the ball to your teammate and he or she will have space and time to work with. If they leave early and try and get across to defend your teammate, you throw the dummy pass and go through the hole.
However, to effectively throw the dummy or really commit your defender, try your best to stay tall on the pass, as you do when sprinting. By ducking down and crouching over, you lose speed and thus the dummy pass won't be as effective because the covering defense will get to you. For an example of how to stay tall, watch Beauden Barrett operate. He's always a threat to run or to pass and it makes life extremely difficult for the defense.