I want to stand up and pump a hill on mountain bike

[JohnV]

If you have a fairly decent bike shop in your area, they should carry the bar ends in stock so you can hold them and get somewhat of a feel for them. I have not used them before, but my friend who is into MTB has them on his bike and uses them all the time. They also help out when you are riding long distances as they give you another position for your hands.