A "spike" is an area where you develop exceptional depth and achievement - something that makes you stand out from other applicants.
Instead of being well-rounded across many areas, a spike means being exceptional in one area. Examples include:
Sophomore year is for exploration and commitment. By end of junior year, you should have tangible results to show.
Document summer accomplishments
Junior Year
Start activity tracking document
Freshman Year
Submit selective summer apps
Sophomore Year
Consider summer job
Freshman Year
Explore passion project
Freshman Year