Stacking is a tactic I introduced to my team last winter because I found that when our D went back for puck retrievals, one of two things would happen.

1. The first D back retrieving the puck would be faced with hard pressure, making a quick pass to their partner on the weaker side. After completing the pass, she continues her momentum toward her teammate, taking the opposing forechecker with them. Typically, what happens is that the weakside D recieves the puck around the same time the strong side D is within feet of them, making the pass irrelevant and often resulting in lost possession and a failed breakout.
2. While the strong side D is retrieving the puck, the weakside D stands in front of the goalie, basically taking themselves out of the play and not being an option for his or her D partner.

In this video, you can see how the Portland Winterhawks are going back to retrieve the puck. Since they have numbers back to help, the weak side D will retreat to the weak side of the ice for a viable "OUT" option for his D partner who is under hard pressure. The centerman or first forward back automatically becomes "stack" player for support. 

In this video, you can see how there are only two defencemen going back into their zone and zero support from the forwards. In this case, you can see how the weakside D scans the ice and moves into the "stack" position, allowing his partner to pick up the puck along the wall, using the net to create separation from the other team's F1. And begins to wheel to the weak side, all while the weak side D becomes an option for a reverse play.

I included this video because it represents 1st reason I mentioned above why I introduced stack plays. As you can see, Josh Morrissey picks up the puck with a lateral support from his D partner and a centerman stacking for support. As Morrissey makes the D-to-D pass, he keeps the momentum towards his D partner. Leaving little to no time for his partner to make the appropriate play. Morrissey continues to skate towards the weak side, taking St. Louis F1 with him. Morrissey is within 10 feet of his partner while the weak side D is receiving this puck, making the D-to-D pass irrelevant. This results in loading the weak side of the ice, becoming outnumbered by the St. Louis Blues, and a failed breakout attempt by the Jets.