Climb aboard the bandwagon

Hello,

Could anyone help me with the following questions?

1. What does "climb aboard the bandwagon immediately, without hedging" mean?

2. What do the words in red mean? I am confused by those "denting for fender", "hammer". It seems that those are metaphors? Could you explain the meanings of them as well as the literal meanings?

Many thanks!

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(from Conditioned Reflex Therapy by Salter)

"Don't try to say anything clever. Simply stick to your emotional guns, and you will find that your answers will be effective. Set up your own issues and stick to them. You can't win on the battleground your brother chooses. If you follow his compass you're sure to lose. Follow your own compass. To change the results you must change your strategy."

After seven sessions, he told me that he felt better than he " can ever remember feeling," and that his wife thanked me. I saw him thereafter twice a year, and then once a year for a "general check-up." He has continued referring persons to me.

This case teaches us the following;
1. When resentment toward a relative is expressed (even if it may not be well-founded), it is good policy to climb aboard the bandwagon immediately, without hedging. This establishes excellent rapport. Once the responsibility for denting the fender has been established, it is possible to explain how to hammer it from underneath, in order to remove the dent. Excitation will even up the inhibition caused by environment.