当驾

• 口(mouth; sound)RADICAL
• 當(equivalent; opportune)PHONETIC

The sound of a bell ringing from an opening makes a "clanging" noise that rings out at just the right moment.

• 馬(horse)RADICAL
• 加(add; press on)PHONO-SEMANTIC

To control a horse, you place a saddle and reins on its back for riding.