Does training a gym that you are a member of make it harder to take over? [duplicate]

I am a member of a level 7 gym. There are 2 Lapras, 2 Snorlax, 1 Arcanine, 1 Vaporeon, and 1 Pidgey. The current prestige of the gym is 25,432 / 30,000. The Arcanine joined and left the prestige at about 20,500. I trained (even though I'm already a member) to push the prestige to 25,432 so far. This sets a higher bar for someone who tries to conquer this gym, right? I'm not quite sure how it works.

Does this make sense? If I continue training and can push prestige above 30,000, then I've opened an extra defender slot, but I can't control who fills it. Someone walking down the street could notice this extra slot and join. I'd hope for a Dragonite, but I'll take any extra person to help defend.

So, does training a gym that I'm a member of help defend it?