You can create transitions between artboards by creating links from layers or hotspots. It's preferable to create links from layers because if you move the layer the link's hit area will move with it. Also if you animate the layer you will need to create a layer link as hotspots don't move. If you want to create a hotspot in an animated layer it's best to create a layer with zero opacity and create a layer link from that. To create a layer link, select a layer and either press Q or press the Layer Link button in the Protowire panel in the bottom right of Sketch. To change the target artboard, select the link and drag the handle near the arrow to another artboard. To delete a link, select it and press the Delete key.
Sometimes you might not be able to create a link from a layer - maybe the layer is nested within a symbol. In which case you can use a hotspot. To create a hotspot link press the Hotspot Link button in the Protowire panel and drag out the hotspot in an artboard. You can apply resizing constraints to hotspots by selecting it and clicking on Sketch's resizing constraints options. You will need to do this so that the hotspot is the correct shape when the prototype's screen resizes. To edit a hotspot link, click anywhere on the link apart from the hotspot rectangle. You can resize a hotspot by selecting the link and dragging on one of the handles.
This will create a link that goes back to the previous screen and reverses the transition. These are useful if you have a navigation bar symbol. You can create a hotspot link on the back text of the symbol and have it work throughout the prototype.
For web prototypes you might want to link to a 3rd party webpage. These links are also useful in native mobile apps to show things like login with Facebook functionality.
Creating links in symbols is a good way to reduce the number of similar links you have to create on each screen. For example you can create one back link in the navigation bar symbol rather than creating a back link on each screen. To create a link in the Symbols page, create the link as normal and the click on the Artboard button at the top. The popover allows you to select the page and artboard you want to link to.
You can create a link from the symbol's artboard and have a link from the symbol's instance override it. For example you might want most symbols to link to one screen but have a few link to another screen.
There is something similar for hotspots. If a hotspot overlaps another hotspot in a symbol the former takes precedence. For example in a navigation bar symbol you might have a back hotspot link on the left. In the artboard that contains the navigation bar symbol you might want one screen to link somewhere else rather than to the previous screen. In which case you can just create a hotspot link that is roughly over the one in the symbol and it will take precedence.
Protowire can make use of Sketch's built-in prototyping. Sketch's links and hotspots will become part of the Protowire prototype, though if a layer has a Sketch link and a Protowire link the Protowire one will be used.