Avira. the Besht taught that improper thoughts should not be avoided. The reason is that he believed that since the entire world is a manifestation of Hashem’s will, and Hashem is purely good, therefore everything in the world is good. This is the basis of the whole idea of klipos, hiding kernels of holiness within them.
The Besht said that if one was in middle of davening, and began having improper thoughts, one should NOT try to remove them from his mind. Instead, he should concentrate on the images, and appreciate the beauty within them. Try to acknowledge that it all comes from Hashem. Since lust is connected with love, if one recognizes that the lust comes from Hashem, concentrating on it will lead to Ahavas Hashem.
Indeed, one of his students, Leib Melamed from Brody, wrote a commentary on the Tur, and he commented that it’s praiseworthy to try and picture inappropriate images while davening, in order to elevate your love of Hashem.
Obviously, the misnagdim strongly opposed this idea. R’ Dovid of Makow, one of the misnagdim, explained something which the Gra said – the Gra came out publicly against chassidim exclaiming “Yakar!” while davening. R’ Dovid said the Gra picked this issue because Yakar has the same letters as Keri. He wouldn’t use the term straight out due to tznius, but he was alluding to the improper ideas they had during davening.