The phrase "bottom position" refers to being in a lower or unfavorable position in a situation or hierarchy. It implies that someone or something is at a disadvantage or in a less desirable state compared to others. Full definition
Once in bottom position, push your chest up reversing the movement. (fitnessandpower.com)
First, set the rails of the rack to just very slightly below the dead hang bottom position of your normal barbell curl. (fitstep.com)
As with pull ups one way to do this is to include static holds on each repetition by holding the top, middle and bottom positions for 5 seconds. (unique-bodyweight-exercises.com)