In aikido Musubi has many meanings. In this instance I will discuss instant contact.
Contact does not simply mean the moment when we feel that "something" is about to happen. Perhaps a stranger has broken our Ma-ai or the body language is such that a threat is perceived indeed we may have come under verbal threat. By maintaining Zanshin and remaining calm and alert in our body and mind we are best prepared to assess the situation.
We must train constantly to be capable of seeing clearly other's intent. When the potential for conflict appears the Aikidoka may move first to pre-empt any attack. Thrust forward at the attacker's face breaking the Ma-ai and drawing his arm up in defence or cut into the hand or foot that is about to attack while driving in an atemi to engage the limbs and destroy the balance. In this decisive manner the Ki Ken Tai Ichi of the attacker shall be broken. While at the instant of Musubi the Aikidoka is at his strongest physically, spiritually and tactically having moved into a Kuzushi.
Musubi is the most decisive and important movement in a technique. We must not allow our technique to stop here.
While studying we shall often pause at the Musubi to check our angle of entry and the imbalance of the uke - this is study.
In a practical situation we cut directly through often unaware of the actual technique used, trusting in spirit, decision, timing, and distance; and balance all elements.
Never fight or struggle with the attacker. Dictate the timing and distancing and cut powerfully through destroying his Ki Ken Tai Ichi
To blend and tie in, to become one, to harmonise.
Distance between uke and nage, harmony of space.
Ki Ken Tai Ichi
Spirit, sword, body as one.
Unbalancing an opponent.