The factors commonly used in the factor-based approach generally have low correlations with the market and with each other. This results from the fact that the factors typically represent what is referred to as a zero (dollar) investment or self-financing investment, in which the underperforming attribute is sold short to finance an offsetting long position in the better-performing attribute.
Institute, CFA. 2020 CFA Program Curriculum Level III Volume 3. CFA Institute, 08/2019. VitalBook file.
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The above paragraph completely confuses me. My question is what’s zero (dollar) investment or self-financing investment; and why this approach can be viewed as selling short to finance long.