As in utsāha उत्साह - Resolve
嗟字門，入諸法嗟字不可得故
Kumarajiva
入蹉字門，悟一切法勇健性不可得故
The strength and power
Xuanzang
TSA because will-power (utsaha) cannot be apprehended.
Conze
The syllable sa [means] all mental phenomena are openings because of the unattainability of the strength (utsāha),
Jayarava