Mum hits with her hands.
When he was little, Regulus didn't understand it. Wouldn't Crucio be so much more effective? He found out later, of course, that the Ministry keeps a record of every spell used in his and his brother's vicinity, even if they themselves weren't the casters.
But Regulus doesn't believe it's just about the Trace. He thinks Mum gets a certain satisfaction out of using manual force—that it releases a tension that would stay pent up inside her if she tried to channel it only through her wand.
Nothing stays pent up inside Mum for long.