The secret of internal force or jin lies in your ability to fang song. Song has three purposes in taijiquan. Firstly, it enables chi to flow smoothly. Secondly, it enables you to neutralize oncoming forces and not get affected by them. And lastly, song enables you to discharge force efficiently by allowing force to flow smoothly from feet to limbs, resulting in a powerful fajin.
Therefore, the next time you do taiji and been asked to fang song, know why you are doing it for. If you know song is a state required for neutralization and fajin, your song will be correct and will not be soft like bean curb that cannot be used for self defense. The relationship between song, strength and jin is like the process of melting iron.
Strength is like iron. It is dull, clumsy and not flexible. A muscular man is very strong but he lacks intelligent force. Whereas jin is like melted iron, it is more refined, polished and fluid. To make the transformation possible, high heat is required to melt the iron. Heat is but an environment, a mean to the end (melting iron) and not the end in itself. Same with song, it is but a quality (an environment) required to change brute strength into powerful and flexible jin.
When you understood this and can connect song to chi and jin, with persistent training and self analysis, you will gradually attain what is called in tai chi chuan as 'iron wrapped in cotton'. You will be able to find hardness within softness and vice versa that makes tai chi chuan so profound and effective.
Song without jin is like bean curb easily overpowered. Strength without song is like a piece of dead wood, easily broken. Jin is like a whip, flexible, powerful and explosive.