This synchronization process usually involves touching the ground with both hands, but with some practice and experience, a person can use earth manipulation magic with any kind of physical contact.
During the fight, Grey wasn't going on the offensive and Killian tried to take advantage of this by sneakily performing a mana synchronization with the ground through his legs. It was an excellent move and judgment that could take any opponent off guard.
'Being good at defense doesn't mean shit, once I break your stance it's game over. I will beat you to-'
"Using earth magic, not a bad plan. But ----" said out loud Grey, with a tremendous speed Grey dashed in front of Killian.
"---Like I will let you do it," said Grey standing close to Killian. Due to mana synchronization, his body couldn't react in time, and also the fact that Grey out of nowhere dashed in front of him, caught him off guard.