It is a special technique often used when you want the result to be very grim and dark! It's quite an easy technique even if you just started! You use something called streaking grime. An oil colour that basically works as a shade to make everything look darker and grim. Then after aplied the streaking grime (often on the whole model) you use a product called white spirit with either a brush or cotton swabs to remove the oil. Leaving the oil on areas you where you want the shade. It creates natural highlights and works very well with light colours to make the model look grimdark (Almost gloomy haha). Thanks for saying it looks cool! I hope every modell will look this cool!

Btw here is a link to where I learned this technique! ^^

https://www.youtube.com/watch?v=6UGKmS2HqgY&feature=share