The object ‘grasped’ by bǎ is generally of definite reference. In our example above, the object bēi zǐ must mean ‘the cup(s)’. It may of course be given an explicit definite reference by the addition of a demonstrative, as in:
Tā bǎ nà zhī /nà xiē bēi zǐ xǐ gān jìng le 。
He washed that cup/those cups.
But one cannot say:
*Tā bǎ yī gè bēi zǐ xǐ gān jìng le 。
because ‘yī gè bēi zǐ’ is of indefinite reference.
The verb must, as was implied above, be followed by a complement. In our example, the verb 洗 xǐ ‘wash’ is complemented by the adjective 干净 gān jìng ‘clean’, without which the sentence would not be complete.
One cannot say:
*Tā bǎ nà gè bēi zǐ xǐ 。
The presence of a complement, especially a long one, in fact necessitates the shift of the object in the first place. One cannot say, for example:
*Tā xǐ dé gān gān jìng jìng (de) bēi zǐ 。
While it is possible for the object to follow a verb with a simple complement:
Tā xǐ gān jìng le nà zhī bēi zǐ 。
nonetheless it feels more natural and rhythmically more balanced to use 把 bǎ.
 Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.