Definition and background:
A device used in marches and piano rags to introduce a contrast in style and break the flow of the composition with a loud and intense musical statement. Also known as a break-up strain, it is normally used to create a break between the repeats of the trio. The break strain reinforces the martial notion of the composition with the break strain often said to represent the battle. The dogfight is a special type of break strain characterized by a musical interaction between instruments or groups of instruments playing a short musical phrases that are followed by a short musical phrases from another group of instruments.
For more information on the music term "Break strain" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary