In the discussion on parallel springs, the effective spring constant is derived using the formula keff = k1 + k2, which assumes equal extension in the springs. This assumption is based on the scenario where two adjacent blocks connected by springs are pulled apart, causing both springs to stretch equally. The equal extension is crucial for the derivation, as it simplifies the analysis of the system's behavior under tension. The rationale is that when the blocks are pulled evenly, the forces acting on both springs are balanced, leading to uniform extension. Understanding this concept is essential for accurately analyzing systems involving parallel springs.