You have got to start thinking in layers, learn about masking, track mattes masks and not be afraid to duplicate layers. Here's one approach.
- Create your text layer
- Add an open mask to the text layer to give you the path of the knife
- Set a keyframe for the mask path and copy it
- Add a graphic of a knife to the timeline
- Set a position keyframe for the knife and paste.
- The knife now follows the same path you drew with the open mask
- Go back to the text layer and duplicate it twice
- Edit the mask on the top copy of the text layer and close it so you only see the top half of the text layer
- Add a white solid to the comp and place it between on the bottom layer
- Set the bottom copy of the text layer as a track matte for the blood splash
- Animate the position of the blood splash layer so that it matches the knife cut
- Add another layer at the bottom to create the dripping blood animation
When you're done you might want to add Bevel Alpha to give the blood some shine and surface tension. This all depends on your design.
Here's what happens. The top half copy of your text layer hides the blood layer. The second copy of the text provides a matte for the blood layer. The bottom copy of the text layer gives your text the same color as the top copy until the animated blood layer covers it up.
This is only one of a couple dozen ways to do the effect. Throwing text animators to make the bottom half of the letters separate as they are cut would be a nice addition.