When you have a multi-layer Photoshop file and you want to set one of the layers to Multiply, Overlay or one of the numerous other Blend Modes, you can click the drop-down menu in the Layers panel, but that can be tedious.
An easier way is to use the keyboard shortcut. Simply have the layer you want to switch Blend Modes on selected and have the Move, Lasso or other tool that doesn’t use Blend Modes (such as the Brush tool) active, and hit Shift + (plus) or Shift – (minus) to cycle through the different Blend Modes.
The shortcut works with tools that DO offer Blend Modes (such as the previously mentioned Brush tool or the Gradient tool) as well.
Incidentally, if you use the Multiply Blend Mode as often as I do, you’ll be happy to know you can simply hit Shift + Option + M to quickly apply it without cycling through it or bothering with the drop-down menu. Other often-used Blend Mode Shortcuts include Shift + Option + S applies Screen, Shift + Option + C applies Color.