You don´t lose Steem Power but Voting Power. SP is your liquid Steem "powered up" and bound to Steemit for at least 3 months and is the basis for how much your votes are worth. Voting Power is decreases by each vote and replenishes itself over time - you get a slider to choose your VP after you got a certain SP value. If you look, there are many articles for newbies here on Steemit, that explain these things way better. :)