Hi!
Is not related to boost or shield, you need to use the special power of the agent.
First of all you need to select the most upgraded agent you have. Then, during a game, collect gold on strikes in row to fill the special power bar on the top. Then, when the power is active, use it pressing space bar. If you don't remember how to activate the special power you could enable the tutorial in the options and follow again.
I hope it helps :)