manasika snanam - bath - bathing with a thought or mind or meditation

 magining the lotus feet of the Lord is on your head and water from his feet is flowing all over the body. 

Manthhra to recite are given bellow.

ॐ अपवित्र: पवित्रो वा सर्वावस्थां गतोअपी वा |
य: स्मरेत पुण्डरीकाक्षं स बाहान्तर : शुचि: ||

meditating on Pundarikaksha cleanes both internally and externally when there are situations one can not bath naturally.

https://www.pakkapatriot.com/11-benefits-and-directions-of-trikala-sandhyavandanam/