If you own multiple funds and one drops 15% than sellin some of the other fund(s) to rebalance the overall asset allocation of your portfolio is not market timing.
but before doing so, evaluate the trading costs to do so, and include a rebalancing plan in your Investment Policy Statement.
"I will rebalance annually by selling all my annual contributions made to my work sponsored index mutual funds plan and transferring them to my self directed index etfs accounts. I am able to sell and transfer once per year for no cost. "
I had to remind myself of this when I considered selling my US index to buy Canadian index recently. No deal.