Not sure what is it that you are asking, but to keep it simple I going to say that any way you decide to do a dip can be very challenging. Doing dips on the bars can be as challenging as doing them on the TRX or the ground or on kettlebells or medicine balls or rings or Smith machine or a bench. You can add weights for more resistance or increase the repetitions, both will affect the difficulty level.
Just want to add to what Harris posted because I think it’s important to mention…. Although tricep dips can be an effective strength exercise for the tricep, they can also cause harm to the shoulder. If this is for yourself or a client, I would make sure there are no shoulder/rotator cuff issues since this exercise can aggravate or worsen a condition.
Adding additional weight (dumbbell/weighted vest)is probably the easiest way to crank up the intensity for dips. I also like some of the dip machines available now – particularly Hammer brand. As others have said, be careful with the movement, especially if you’re adding additional resistance. One ugly rep can be pretty rough on shoulders – been there, done that!
As I have asked in response to many such questions. Why?
What application in real life are you planning to enhance by making the dip exercise more challenging? Why not find a movment that engages the extension of the arm in a manner that is safer and has more real world applications? Does your client need to repeatedly climb out of a manhole with a heavy tool belt on? Have you spent anytime evaluating the exercise and comparing it to other movements that could be more appropriate?
I might use dips with a client and seek to increase the challenge for them if they were a competitive gymnast. Otherwise, it would just be a simple dip movement. But I spend a lot of time working post rehab for helping to fix a lot of the damage that is done by exceeding the reasonable manner in which to exercise, so I might just be jaded. Or sincerely trying to follow our code of ethics and be safe and responsible in providing clients with instruction.