Depala, Pilot Exemplar
Legendary Creature — Dwarf Pilot
Other Dwarves you control get +1/+1.
Each Vehicle you control gets +1/+1 as long as it's a creature.
Whenever Depala becomes tapped, you may pay 
. If you do, reveal the top X cards of your library, put all Dwarf and Vehicle cards from among them into your hand, then put the rest on the bottom of your library in a random order.
Greg Opalinski
Kaladesh Remastered (KLR)
• #192Rulings
Depala, Pilot Exemplar's last ability is a triggered ability, not an activated ability. It doesn't allow you to tap Depala whenever you want; rather, you need some other way of tapping it, such as by attacking.
(wotc • 2016-09-20)
For the ability to trigger, Depala has to actually change from untapped to tapped. If an effect attempts to tap Depala, but it was already tapped at the time, this ability won't trigger.
(wotc • 2016-09-20)
Rulings
Depala, Pilot Exemplar's last ability is a triggered ability, not an activated ability. It doesn't allow you to tap Depala whenever you want; rather, you need some other way of tapping it, such as by attacking.
(wotc • 2016-09-20)
For the ability to trigger, Depala has to actually change from untapped to tapped. If an effect attempts to tap Depala, but it was already tapped at the time, this ability won't trigger.
(wotc • 2016-09-20)